Optical scanning system with correction

ABSTRACT

A scanning system device has a predetermined aberration as it scans or switches light along selected optical paths. A deformable membrane receives the light and introduces an inverse “aberration” that offsets that of the scanning system. In one embodiment the scanning system includes a torsion arm that supports an oscillatory body. The torsion arm and/or body can be machined from metal, micromachined in silicon or formed in a variety of other ways. Alternatively, the scanning system may include a rotating polygonal scanner or other type of optical scanner. In another approach, an optical switch replaces the scanner.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.09/400,350, filed Sep. 20, 1999, which is a continuation-in-part of U.S.patent application Ser. No. 09/129,252, filed Aug. 5, 1998, issued U.S.Pat. No.6,140,979.

TECHNICAL FIELD

The present invention relates to scanned light devices and, moreparticularly, to scanned light beam displays and imaging devices forviewing or collecting images.

BACKGROUND OF THE INVENTION

A variety of techniques are available for providing visual displays ofgraphical or video images and for acquiring images. In many displayapplications, cathode ray tube type displays (CRTs), such as televisionsand computer monitors produce images for viewing. Such devices sufferfrom several limitations. For example, CRTs are bulky and consumesubstantial amounts of power, making them undesirable for portable orhead-mounted applications.

Matrix addressable displays, such as liquid crystal displays and fieldemission displays, may be less bulky and consume less power. However,typical matrix addressable displays utilize screens that are severalinches across. Such screens have limited use in head mountedapplications or in applications where the display is intended to occupyonly a small portion of a user's field of view. Such displays have beenreduced in size, at the cost of increasingly difficult processing andlimited resolution or brightness. Also, improving resolution of suchdisplays typically requires a significant increase in complexity.

Devices for acquiring images, such as cameras and scanning microscopesmay have similar constraints. For example, small cameras often use CCDarrays to convert light energy to electrical signals. In highresolutions systems, the CCD array can be complex. Full color CCDapplications can add further complexity.

One approach to overcoming many limitations of conventional displays orimage capture devices is a scanned beam approach, such as that describedfor displays in U.S. Pat. No. 5,467,104 of Furness et al., entitledVIRTUAL RETINAL DISPLAY, which is incorporated herein by reference or asdescribed for image capture in U.S. Pat. No. 5,742,419 to Dickensheetset al., entitled MINIATURE SCANNING CONFOCAL MICROSCOPE, which isincorporated herein by reference.

As shown diagrammatically in FIG. 1, in one embodiment of a scanned beamdisplay 40, a scanning source 42 outputs a scanned beam of light that iscoupled to a viewer's eye 44 by a beam combiner 46. In some scanneddisplays, the scanning source 42 includes a scanner, such as scanningmirror or acousto-optic scanner, that scans a modulated light beam ontoa viewer's retina. In other embodiments, the scanning source may includeone or more light emitters that are rotated through an angular sweep.

The scanned light enters the eye 44 through the viewer's pupil 48 and isimaged onto the retina 59 by the cornea. In response to the scannedlight the viewer perceives an image. In another embodiment, the scannedsource 42 scans the modulated light beam onto a screen that the viewerobserves. One example of such a scanner suitable for either type ofdisplay is described in U.S. Pat. No. 5,557,444 to Melville et al.,entitled MINIATURE OPTICAL SCANNER FOR A TWO-AXIS SCANNING SYSTEM, whichis incorporated herein by reference.

Sometimes such displays are used for partial or augmented viewapplications. In such applications, a portion of the display ispositioned in the user's field of view and presents an image thatoccupies a region 43 of the user's field of view 45, as shown in FIG.2A. The user can thus see both a displayed virtual image 47 andbackground information 49. If the background light is occluded, theviewer perceives only the virtual image 47, as shown in FIG. 2B.

One difficulty that may arise with such displays is raster pinch, aswill now be explained with reference to FIGS. 3-5. As showndiagrammatically in FIG. 3, the scanning source 42 includes an opticalsource 50 that emits a beam 52 of modulated light. In this embodiment,the optical source 50 is an optical fiber that is driven by one or morelight emitters, such as laser diodes (not shown). A lens 53 gathers andfocuses the beam 52 so that the beam 52 strikes a turning mirror 54 andis directed toward a horizontal scanner 56. The horizontal scanner 56 isa mechanically resonant scanner that scans the beam 52 periodically in asinusoidal fashion. The horizontally scanned beam then travels to avertical scanner 58 that scans periodically to sweep the horizontallyscanned beam vertically. For each angle of the beam 52 from the scanners58, an exit pupil expander 62 converts the beam 52 into a set of beams63. Eye coupling optics 60 collect the beams 63 and form a set of exitpupils 65. The exit pupils 65 together act as an expanded exit pupil forviewing by a viewer's eye 64. One such expander is described in U.S.Pat. No. 5,701,132 of Kollin et al., entitled VIRTUAL RETINAL DISPLAYWITH EXPANDED EXIT PUPIL, which is incorporated herein by reference.

One skilled in the art will recognize that, for differing applications,the exit pupil expander 62 may be omitted, may be replaced orsupplemented by an eye tracking system, or may have a variety ofstructures, including diffractive or refractive designs. For example,the exit pupil expander 62 may be a planar or curved structure and maycreate any number or pattern of output beams in a variety of patterns.Also, although only three exit pupils are shown in FIG. 3, the number ofpupils may be almost any number. For example, in some applications a 15by 15 array may be suitable.

Returning to the description of scanning, as the beam scans through eachsuccessive location in the beam expander 62, the beam color andintensity is modulated in a fashion to be described below to form arespective pixel of an image. By properly controlling the color andintensity of the beam for each pixel location, the display 40 canproduce the desired image.

As the beam is scanned, a variety of optical effects may affect thequality of the displayed or captured image. For example, millimeterscale optical instruments typically require lens elements in theintermediate size range of several hundred micrometer clear apertures.Although considerable progress has been made in the fabrication ofsingle refractive microlenses in this regime, the construction ofhigh-power, high-resolution systems of refractive microlenses capable ofdiffraction limited imagery over a large field of view remains aconsiderable technological challenge.

When attempting to form an image with a simple lens (one that is notfully corrected), one observes aberrations in the image that depend onthe location in the field-of-view. The optical surface that worksoptimally for resolving a point in the center of the field is usuallynot quite the same as the optical surface that optimally resolves apoint off to one side in the field-of-view. To solve this problem, thelens designer typically adds additional lens surfaces until the systemis fully corrected throughout the field-of-view. This can addconsiderable complexity for the micro-optical instrument, and in manycases a practical design may not be realized. If, however, the designconstraints are reduced so that the lens design can be optimized for oneparticular position in the image field, a much simpler lens could beused; however, the optimal lens would usually be slightly different foreach pixel. Array based imaging systems that image all of the pixels inthe field simultaneously therefore use lens systems fully corrected tothe desired level of performance.

Considerable effort has been devoted to developing the theory anddevices for adaptive optics for the correction and optimization ofimaging systems such as telescopes and antennas. Much of that work hasfocused on random aberrations from such external sources as atmosphericturbulence, and the surfaces required to compensate such fluctuationsare highly variable. Adaptive mirrors for these applications typicallyhave tens or hundreds of actuators in an array pattern, often with athin continuous membrane over the surface of the array. Control of sucha surface often uses local wavefront sensing and feedback control, andthese surfaces typically are for slowly varying aberrations.

In some examples of such an approach continuous membrane micromachinedadaptive mirrors are use for correction. Such approaches are describedin Gleb Vdovin and P. M. Sarro, “Flexible mirror micromachined insilicon,” Applied Optics vol. 34 no. 16, pp 2968-2972, 1995; P.K.C. Wangand F. Y. Hadeagh, “Computation of static shapes and voltages formicromachined deformable mirrors with nonlinear electrostaticactuators,” JMEMS vol. 5 no. 5, pp 205-220, 1996; Thomas G. Bifano et.al., “Continuous-membrane surface micromachined silicon deformablemirror,” Optical Engineering, vol. 36 no. 5, pp 1354-1359, 1997; LindaM. Miller, Michael L. Agronin, Randall K. Bartman, William J. Kaiser,Thomas W. Kenny, Robert L. Norton and Erika C. Vote, “Fabrication andcharacterization of a micromachined deformable mirror for adaptiveoptics applications,” SPIE 1945, 1993; and Adrian M. Michalicek, NatalieClark, John H. Comtois, Heather K. Schriner, “Design and simulation ofadvanced surface micromachined micromirror devices for telescopeadaptive optics applications,” SPIE 3353, pp. 805-815, 1998 each ofwhich is incorporated herein by reference.

Returning to the general description of scanning, simplified versions ofthe respective waveforms of the vertical and horizontal scanners areshown in FIG. 4. In the plane 66 (FIG. 3), the beam traces the pattern68 shown in FIG. 5. Though FIG. 5 shows only eleven lines of image, oneskilled in the art will recognize that the number of lines in an actualdisplay will typically be much larger than eleven. As can be seen bycomparing the actual scan pattern 68 to a desired raster scan pattern69, the actual scanned beam 68 is “pinched” at the outer edges of thebeam expander 62. That is, in successive forward and reverse sweeps ofthe beam, the pixels near the edge of the scan pattern are unevenlyspaced. This uneven spacing can cause the pixels to overlap or can leavea gap between adjacent rows of pixels. Moreover, because the imageinformation is typically provided as an array of data, where eachlocation in the array corresponds to a respective position in the idealraster pattern 69, the displaced pixel locations can cause imagedistortion.

Further optical aberrations in the optical train may produce pixelnon-uniformities, distortion, or other artifacts that may reduce theappearance or degrade the performance of a scanned display or imagecapture device.

SUMMARY OF THE INVENTION

A display includes a primary scanning mechanism that simultaneouslyscans beams of light. In a one embodiment of a display according to theinvention, the scanning mechanism scans the beams along substantiallycontinuous scan paths where each beam defines a discrete “tile” of animage. In the preferred embodiment, the scanning mechanism includes amirror that pivots to sweep the beams horizontally.

In this tiled embodiment, optical sources are aligned to provide thebeams of light to the scanning mechanism from respective input angles.The input angles are selected such that the scanning mechanism sweepseach beam of light across a respective distinct region of an imagefield. Because the respective regions are substantially non-overlapping,each beam of light generates a substantially spatially distinct regionof the image. The respective regions are immediately adjacent or mayoverlap slightly, so that the spatially distinct regions are “tiled” toform a contiguous image. Because movement of the mirror producesmovement of all of the beams, the display produces each of the spatiallyseparate regions simultaneously. As described above, the scan angle θand the mirror dimensions determine the number of pixels drawn for eachbeam. The total number of pixels in a line can thus substantially equalthe number of pixels for each beam multiplied by the number of beams.

A scanning system device has a predetermined aberration as it scans orswitches light along selected optical paths. A deformable membranereceives the light and introduces an inverse “aberration” that offsetsthat of the scanning system. In one embodiment the scanning systemincludes a torsion arm that supports an oscillatory body. The torsionarm and/or body can be machined from metal, micromachined in silicon orformed in a variety of other ways. Alternatively, the scanning systemmay include a rotating polygonal scanner or other type of opticalscanner. In another approach, an optical switch replaces the scanner.

In one embodiment, the scanning mechanism scans in a generally rasterpattern with a horizontal component and a vertical component. Amechanically resonant scanner produces the horizontal component byscanning the beam sinusoidally. A non-resonant or semi-resonant scannertypically scans the beam vertically with a substantially constantangular speed.

In one embodiment, the scanning mechanism includes a biaxialmicroelectromechanical (MEMs) scanner. The biaxial scanner uses a singlemirror to provide both horizontal and vertical movement of each of thebeams. In one embodiment, the display includes a buffer that stores dataand outputs the stored data to each of the optical sources. A correctionmultiplier provides correction data that adjusts the drive signals tothe optical sources in response to the stored data. The adjusted drivesignals compensate for variations in output intensity caused by patterndependent heating.

In one embodiment, the MEMs scanner is a resonant scanner that has acharacteristic resonant frequency. Where the resonant frequency does notmatch the rate at which image data is supplied, data may be clocked intoand out of the buffer at different rates.

In another embodiment of the MEMs scanner, the MEMs scanner may have atunable resonant frequency that can be adjusted to conform to the rateat which image data is provided. In one embodiment of such a MEMsscanner, a primary oscillatory body carries secondary masses that can beremoved or added to, thereby shifting the amount of mass pivoting abouta scan axis. The shifting amount of mass changes the resonant frequencyand can be established by laser ablation, or other mass removal oraddition techniques. By monitoring movement of the oscillatory body andcomparing the monitored movement to the desired scanning frequency, acontrol circuit can control an automated fabrication system to tuneseveral MEMs scanner.

In one embodiment, an imager acquires images in tiles by utilizing twoseparate detector and optical source pairs. One embodiment of the imagerincludes LEDs or lasers as the optical sources, where each of theoptical sources is at a respective wavelength. The scanning assemblysimultaneously directs light from each of the optical sources torespective regions of an image field. For each location in the imagefield, each of the detectors selectively detects light at thewavelength, polarization, or other characteristic of its correspondingsource, according to the reflectivity of the respective location. Thedetectors output electrical signals to decoding electronics that storedata representative of the image field.

In one embodiment, the imager includes a plurality of detector/opticalsource pairs at each of red, green, and blue wavelength bands. Each pairoperates at a respective wavelength within its band. For example, afirst of the red pairs operates at a first red wavelength and a secondof the red pairs operates at a second red wavelength different from thefirst.

In one embodiment, a pair of optical sources alternately feed a singlescanner from different angles. During forward sweeps of the scanner, afirst of the sources emits light modulated according to one half of aline. During the return sweep, the second source emits light modulatedaccording to the second half of the line. Because the second sweep is inthe opposite direction from the first, data corresponding to the secondhalf of the line is reversed before being applied to the second sourceso that light from the second source is modulated to write the secondhalf of the line in reverse.

In one embodiment of the alternate feeding approach, a single lightemitter feeds an input fiber that is selectively coupled to one of twoseparate fibers by an optical switch. During forward sweeps, the opticalswitch couples the input fiber to a first of the separate fibers so thatthe first separate fiber forms the first optical source. During reversesweep, the optical switch feeds the second separate fiber so that thesecond separate fiber forms the second source. This embodiment thusallows a single light emitter to provide light for both optical sources.

The alternate feeding approach can be expanded to write more than justtwo tiles. In one approach, the input fiber is coupled to four fibers bya set of optical switches, where each fiber feeds the scanning assemblyfrom a respective angle. The switches are activated according to thedirection of the sweep and according to the tracked location of theuser's vision. For example, when the user looks at the top half of theimage, a first fiber, aligned to produce an image in the upper left tilefeeds the scanner during the forward sweeps. A second fiber, aligned toproduce an upper right tile feeds the scanner during reverse sweeps.When the user looks at the lower half of the image, a third fiber,aligned to produce the lower left tile, feeds scanner during forwardsweeps. A fourth fiber, aligned to produce the lower right tile, feedsthe scanner during reverse sweeps.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagrammatic representation of a display aligned to aviewer's eye.

FIG. 2A is a combined image perceived by a user resulting from thecombination of light from an image source and light from a background.

FIG. 2B is an image perceived by a user from the display of FIG. 1 wherethe background light is occluded.

FIG. 3 is a diagrammatic representation of a scanner and a user's eyeshowing bi-directional scanning of a beam and coupling to the viewer'seye.

FIG. 4 is a signal-timing diagram of a scan pattern scanner in thescanning assembly of FIG. 3.

FIG. 5 is a signal position diagram showing the path followed by thescanned beam in response to the signals of FIG. 4, as compared to adesired raster scan path.

FIG. 6 is a diagrammatic representation of a display according to theone embodiment invention including dual light beams.

FIG. 7 is an isometric view of a head-mounted scanner including atether.

FIG. 8 is a diagrammatic representation of a scanning assembly withinthe scanning display of FIG. 6, including a correction mirror.

FIG. 9 is an isometric view of a horizontal scanner and a verticalscanner suitable for use in the scanning assembly of FIG. 8.

FIG. 10 is a diagrammatic representation of scanning with two inputbeams, showing slightly overlapped tiles.

FIG. 11 is a top plan view of a biaxial scanner showing four feeds atspatially separated locations.

FIG. 12 is a diagrammatic representation of four tiles produces by thefour feed scanner of FIG. 11.

FIG. 13 is a schematic of a system for driving the four separate feedsof FIG. 11, including four separate buffers.

FIG. 14 is a signal-timing diagram comparing a ramp signal with adesired signal for driving the vertical scanner.

FIG. 15 is a signal timing diagram showing positioning error andcorrection for the vertical scanning position.

FIG. 16 is a side cross sectional view of a piezoelectric correctionscanner.

FIG. 17A is a top plan view of a microelectromechanical (MEMs)correction scanner.

FIG. 17B is a side cross-sectional view of the MEMs correction scannerof FIG. 17A showing capacitive plates and their alignment to thescanning mirror.

FIG. 18 shows corrected scan position using a sinusoidally drivenscanner through 90% of the overall scan.

FIG. 19 shows an alternative embodiment of a reduced error scanner wherescan correction is realized by adding a vertical component to thehorizontal mirror.

FIG. 20 is a position diagram showing the scan path of a beam deflectedby the scanner of FIG. 19.

FIG. 21 is a diagrammatic view of a scanning system, including a biaxialmicroelectromechanical (MEMs) scanner and a MEMs correction scanner.

FIG. 22 is a diagrammatic view of a correction scanner that shifts aninput beam by shifting the position or angle of the input fiber.

FIG. 23 is a diagrammatic view of a correction scanner that includes anelectro-optic crystal that shifts the input beam in response to anelectrical signal.

FIG. 24 is a diagrammatic view of an imager that acquires external lightfrom a target object.

FIG. 25 is a diagrammatic view of an alternative embodiment of theimager of FIG. 24 that also projects a visible image.

FIG. 26 is a signal timing diagram showing deviation of a sinusoidalscan position versus time from the position of a linear scan.

FIG. 27 is a diagram showing diagrammatically how a linear set of countscan map to scan position for a sinusoidally scan.

FIG. 28 is a system block diagram showing handling of data to store datain a memory matrix while compensating for nonlinear scan speed of theresonant mirror.

FIG. 29 is a block diagram of a first system for generating an outputclock to retrieve data from a memory matrix while compensating fornonlinear scan speed of the resonant mirror.

FIG. 30 is a block diagram of an alternative embodiment of the apparatusof FIG. 29 including pre-distortion.

FIG. 31 is a detail block diagram of a clock generation portion of theblock diagram of FIG. 29.

FIG. 32 is a representation of a data structure showing datapredistorted to compensate for vertical optical distortion.

FIG. 33 is a top plan view of a MEMs scanner including structures forelectronically controlling the center of mass of each mirror half.

FIG. 34 is a top plan view of the MEMs scanner of FIG. 32 showingflexing of protrusions in response to an applied voltage.

FIG. 35 is a top plan view of a MEMs scanner including comb structuresfor laterally shifting the center of mass of each mirror half.

FIG. 36 is a side cross sectional view of a packaged scanner includingelectrically controlled outgassing nodules.

FIG. 37 is a top plan view of a MEMs mirror including selectivelyremovable tabs for frequency tuning.

FIG. 38A is a top plan view of a substrate including an array ofscanners having removable material for on-substrate frequency tuning.

FIG. 38B is a top plan view of a uniaxial scanner having a block of amigrating impurity on its torsion arms for selective frequency tuning.

FIG. 39 is a diagrammatic view of a four source display showing overlapof scanning fields with optical sources.

FIG. 40 is a diagrammatic view of a four source display with smallturning mirrors and offset optical sources.

FIG. 41 is a diagrammatic view of the display of FIG. 40 showing beampaths with the small turning mirrors and a common curved mirror.

FIG. 42 is a diagrammatic view of a single emitter display includingswitched optical fibers each feeding a separate tile.

FIG. 43 is a diagrammatic view of a display including four separatefibers feeding a scanner through a set of optical switches in responseto a detected gaze direction to produce four separate tiles.

FIG. 44 is a diagram of a calculated wave aberration corrected byreflection from a membrane with surface shape figure s(x,y).

FIG. 45 is a diagram showing coordinates for the pupil plane and imageplane.

FIG. 46 is a set of graphs showing differing types of aberrations.

FIG. 47 is a side cross-sectional view of an ACM device showing layersof material.

FIG. 48 is a a photomicrograph of a circular ACM, 200 μm in diameterwith 25% duty width support ring.

FIG. 49 is a photomicrograph of a rectangular ACM, 250 μm long by 500 μmwide, supported along long sides and free along short sides.

FIGS. 50 a-d are calculated effects of edge support stiffness and thepresence of gold on spherical aberration for 200 μm diameter membraneswhere curves correspond to near-maximum deflections for each device.

FIG. 51 is a calculated surface plot of fabricated polysilicon (no gold)ACM for astigmatism compensation where the surface figure is computedfrom interferometric microscope images.

FIGS. 52 a-h are curve fits for calculated effects of edge supportstiffness and the presence of gold on spherical aberration for 200 μmdiameter membranes where curves correspond to near-maximum deflectionsfor each device.

FIG. 53 is a diagrammatic view of a laser beam scanner with ACM devicesfor astigmatism and field curvature correction.

FIG. 54 includes photographic images of laser beam spot at the imageplane in (a) center of the field of view with no compensation; (b) 7.5°beam scan angle with no compensation; (c) 7.5° beam scan angle withdefocus compensation; and (d) 7.5° beam scan angle with defocus andastigmatism compensation

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 6, a scanned beam display 70 according to oneembodiment of the invention is positioned for viewing by a viewer's eye72. While the display 70 is presented herein is scanning light into theeye 72, the structures and concepts described herein can also be appliedto other types of displays, such as projection displays that includeviewing screens. Moreover, the invention is not limited to scanned beamdisplays. For example, as will be described below, image capture devicesor optical switches may also be within the scope of the invention.

The display 70 includes four principal portions, each of which will bedescribed in greater detail below. First, control electronics 74 provideelectrical signals that control operation of the display 70 in responseto an image signal V_(IM) from an image source 76, such as a computer,television receiver, videocassette player, DVD player, remote sensor, orsimilar device.

The second portion of the display 70 is a light source 78 that outputsmodulated light beams 80, each having a modulation corresponding toinformation in the image signal V_(IM). The light source 78 may utilizecoherent light emitters, such as laser diodes or microlasers, or may usenon-coherent sources such as light emitting diodes. Also, the lightsource 78 may include directly modulated light emitters such as thelight emitting diodes (LEDs) or may include continuous light emittersindirectly modulated by external modulators, such as acousto-opticmodulators.

The third portion of the display 70 is a scanning assembly 82 that scansthe modulated beams 80 through two-dimensional scanning patterns, suchas raster patterns. The scanning assembly 82 preferably includes aperiodically scanning mirror or mirrors as will be described in greaterdetail below with reference to FIGS. 3-4, 8, 11, 19-22.

Lenses 84, 86 positioned on opposite sides of the scanning assembly 82act as imaging optics that form the fourth portion of the display 70.The lenses 86 are cylindrical graded index (GRIN) lenses that gather andshape light from the light source 78. Where the light source 78 includesoptical fibers that feed the lenses 86, the lenses 86 may be bonded toor integral to the fibers. Alternatively, other types of lenses, such asdoublets or triplets, may form the lenses 86. Also, other types ofoptical elements such as diffractive elements may be used to shape andguide the light. Regardless of the type of element, the overall opticaltrain may incorporate polarization sensitive materials, chromaticcorrection, or any other optical technique for controlling the shape,phase or other characteristics of the light.

The lens 84 is formed from a curved, partially transmissive mirror thatshapes and focuses the scanned beams 80 approximately for viewing by theeye 72. After leaving the lens 84, the scanned beams 80 enter the eye 72through a pupil 90 and strike the retina 92. As each beam of scannedmodulated light strikes the retina 92, the viewer perceives a respectiveportion of the image as will be described below.

Because the lens 84 is partially transmissive, the lens 84 combines thelight from the scanning assembly 82 with the light received from abackground 89 to produce a combined input to the viewer's eye 72.Although the background 89 is presented herein as a “real-world”background, the background light may be occluded or may be produced byanother light source of the same or different type. One skilled in theart will recognize that a variety of other optical elements may replaceor supplement the lenses 84, 86. For example, diffractive elements suchas Fresnel lenses may replace either or both of the lenses 84, 86.Additionally, a beamsplitter and lens may replace the partiallytransmissive mirror structure of the lens 84. Moreover, various otheroptical elements, such as polarizers, color filters, exit pupilexpanders, chromatic correction elements, eye-tracking elements, andbackground masks may also be incorporated for certain applications.

Although the elements of FIG. 6 are presented diagrammatically, oneskilled in the art will recognize that the components are typicallysized and configured for the desired application. For example, where thedisplay 70 is intended as a mobile personal display the components aresized and configured for mounting to a helmet or similar frame as ahead-mounted display 70, as shown in FIG. 7. In this embodiment, a firstportion 171 of the display 70 is mounted to a head-borne frame 174 and asecond portion 176 is carried separately, for example in a hip belt. Theportions 174, 176 are linked by a fiber optic and electronic tether 178that carries optical and electronic signals from the second portion tothe first portion. An example of a fiber-coupled scanner display isfound in U.S. Pat. No. 5,596,339 of Furness et al., entitled VIRTUALRETINAL DISPLAY WITH FIBER OPTIC POINT SOURCE which is incorporatedherein by reference.

An exemplary embodiment of the scanning assembly 82 will be describednext with reference to FIG. 8. The scanning assembly 82 includes severalcomponents that correspond to the scanning source 42 of FIG. 3, wherecomponents common to the scanning assembly 82 and scanning source 42 arenumbered the same. Additionally, only central rays 55 are presented forthe beams 52 for clarity of presentation.

In this embodiment, a pair of fibers 50 emit light from the lightsources 78 (not shown) and the lens 84 is represented as a commonrefractive lens rather than as a partially transmissive mirror. Unlikethe scanning source 42 of FIG. 3, the scanning assembly 82 includes anactive correction mirror 100 that can pivot to scan the light beam 80along the vertical axis. As will be explained below, the correctionmirror 100 produces a varying corrective shift along the vertical axisduring each sweep (forward or reverse) of the horizontal scanner 56. Thecorrective shift offsets vertical movement of the beams 80 caused by thevertical scanner 58 to reduce the overall deviation of the scanningpattern from the desired pattern shown in broken lines in FIG. 5. Thecorrection mirror 100 carries a deformable membrane 180 that cancompensate for aberrations in the optical system or optical path lengthvariations due to the scanning systems.

Before describing the structure and effects of the correction mirror 100and the deformable membrane 180 and the relative timing of the varioussignals, exemplary embodiments of mechanically resonant scanner 200, 220suitable for use as the horizontal scanner 56 and vertical scanner 58will be described with reference to FIG. 9.

The principal scanning component of the horizontal scanner 200 is amoving mirror 202 mounted to a spring plate 204. The dimensions of themirror 202 and spring plate 204 and the material properties of thespring plate 204 have a high Q with a natural oscillatory (“resonant”)frequency on the order of 1 -100 kHz, where the selected resonantfrequency depends upon the application. For VGA quality output with a 60Hz refresh rate and no interlacing, the resonant frequency is preferablyabout 15-20 kHz. As will be described below, the selected resonantfrequency or the achievable resolution may be changed through the use ofa plurality of feeds.

A ferromagnetic material mounted with the mirror 202 is driven by a pairof electromagnetic coils 206, 208 to provide motive force to mirror 202,thereby initiating and sustaining oscillation. The ferromagneticmaterial is preferably integral to the spring plate 204 and body of themirror 202. Drive electronics 218 provide electrical signals to activatethe coils 206, 208, as described above. Responsive to the electricalsignals, the coils 206, 208 produce periodic electromagnetic fields thatapply force to the ferromagnetic material, thereby causing oscillationof the mirror 202. If the frequency and phase of the electric signalsare properly synchronized with the movement of the mirror 202, themirror 202 oscillates at its resonant frequency with little powerconsumption.

The vertical scanner 220 is structured very similarly to the resonantscanner 200. Like the resonant scanner 201, the vertical scanner 220includes a mirror 222 driven by a pair of coils 224, 226 in response toelectrical signals from the drive electronics 218. However, because therate of oscillation is much lower for vertical scanning, the verticalscanner 220 is typically not resonant. The mirror 222 receives lightfrom the horizontal scanner 201 and produces vertical deflection atabout 30-100 Hz. Advantageously, the lower frequency allows the mirror222 to be significantly larger than the mirror 202, thereby reducingconstraints on the positioning of the vertical scanner 220. The detailsof virtual retinal displays and mechanical resonant scanning aredescribed in greater detail in U.S. Pat. No. 5,467,104, of Furness III,et al., entitled VIRTUAL RETINAL DISPLAY which is incorporated herein byreference.

One skilled in the art will recognize a variety of other structures thatmay scan a light beam through a generally raster pattern. For example,spinning polygons or galvanometric scanners may form either or both ofthe scanners 56, 58 in some applications.

In another embodiment, a bi-axial microelectromechanical (MEMs) scannermay provide the primary scanning. Some such scanners are described inU.S. Pat. No. 5,629,790 to Neukermanns et al., entitled MICROMACHINEDTORSIONAL SCANNER, which is incorporated herein by reference. While thescanner of the '790 patent is the presently preferred embodiment, avariety of other MEMs scanners may also be appropriate for certainapplications. For example, surface micromachined biaxial scanners andother MEMs scanners have been described by various authors.

Like the scanning system described above, the horizontal components ofthe MEMs scanners are typically defined by mechanical resonances oftheir respective structures, as will be described in greater detailbelow with reference to FIGS. 17A-B and 21. Like the two scanner systemdescribed above with reference to FIGS. 3 and 8, such biaxial scannersmay suffer similar raster pinch problems due to movement along theslower scan axis during sweeps along the faster scan axis. Otherscanning approaches may also apply. For example, acousto-optic scanners,electro-optic scanners, spinning polygons, or some combination ofscanning approaches can provide the scanning function. Some of theseapproaches may not require pinch correction.

Returning to FIGS. 6, 8 and 9, the fibers 50 output light beams 80 thatare modulated according to the image signal from the drive electronics218. At the same time, the drive electronics 218 activate the coils 206,208, 224, 226 to oscillate the mirrors 202, 222. The modulated beams oflight strike the oscillating horizontal mirror 202 (of the horizontalscanner 56), and are deflected horizontally by an angle corresponding tothe instantaneous angle of the mirror 202. The deflected beams thenstrike the vertical mirror 222 (of the vertical scanner 58) and aredeflected at a vertical angle corresponding to the instantaneous angleof the vertical mirror 222. After expansion by the beam expander 62, thebeams 52 pass through the lens 84 to the eye. As will also be describedbelow, the modulation of the optical beams is synchronized with thehorizontal and vertical scans so that, at each position of the mirrors,the beam color and intensity correspond to a desired virtual image. Eachbeam therefore “draws” a portion of the virtual image directly upon theuser's retina.

One skilled in the art will recognize that several components of thescanning assembly 82 have been omitted from the FIG. 9 for clarity ofpresentation. For example, the horizontal and vertical scanners 200, 220are typically mounted to a frame. Additionally, lenses and other opticalcomponents for gathering, shaping, turning, focusing, or collimating thebeams 80 have been omitted. Also, no relay optics are shown between thescanners 200, 220, although these may be desirable in some embodiments.Moreover, the scanner 200 typically includes one or more turning mirrorsthat direct the beam such that the beam strikes each of the mirrors aplurality of times to increase the angular range of scanning. Further,in some embodiments, the scanners 200, 220 are oriented such that thebeam can strike the scanning mirrors a plurality of times without aturning mirror.

Turning to FIGS. 10 and 11, the effect of the plurality of beams 80 willnow be described. As is visible in FIG. 10, two fibers 50 emitrespective light beams 80. The GRIN lenses 86 gather and focus the beams80 such that the beams 80 become converging beams 80A, 80B that strike acommon scanning mirror 1090.

For clarity of presentation, the embodiment of FIG. 10 eliminates themirror 84, as is desirable in some applications. Also, the embodiment ofFIG. 10 includes a single mirror 1090 that scans biaxially instead ofthe dual mirror structure of FIG. 9. Such a biaxial structure isdescribed in greater detail below with reference to FIGS. 11, 17A-B and21. One skilled in the art will recognize that a dual mirror system mayalso be used, though such a system would typically involve a morecomplex set of ray traces and more complex compensation for differingoptical path lengths.

Also, although the fibers 50 and lenses 84 of FIG. 10 appear positionedin a common plane with the scanning mirror 1090, in many applications,it may be desirable to position the fibers 50 and lenses 84 off-axis, asis visible in FIG. 11. Moreover, where four fiber/lens pairs are used,as in FIG. 11, a beam splitter or other optical elements can allow thefiber/lens pairs to be positioned where they do not block beams 80A-Dfrom other fiber/lens pairs. Alternatively, other approaches, such assmall turning mirrors can permit repositioning of the fiber/lens pairsin non-blocking positions with little effect on the image quality. Suchapproaches are described in greater detail below with reference to FIGS.11 and 38A-41.

After exiting the lens 86, the first beam 80A strikes the scanningmirror 1090 and is reflected toward an image field 1094. The second beam80B is also reflected by the scanning mirror 1090 toward the image field1094. As shown by the ray tracing of FIG. 10, the horizontal position ofthe beams 80A-B in the image field 1094 will be functions of the angulardeflection from the horizontal scanner 56 and the position andorientation of the lens 86 and fiber 50.

At the image field 1092, the first beam 80A illuminates a first region1092 of the image field 1094 and the second beam 80B illuminates asecond region 1096 that is substantially non-overlapping with respect tothe first region 1092. To allow a smooth transition between the tworegions 1092, 1096, the two regions 1092, 1096 overlap slightly in asmall overlap region 1098. Thus, although the two regions aresubstantially distinct, the corresponding image portions may be slightly“blended” at the edges, as will be described below with reference toFIGS. 12 and 13.

While only two beams 80A-B are visible in FIG. 10, more than twofiber/lens pairs can be used and the fiber/lens pairs need not becoplanar. For example, as can be seen in FIG. 11, four separate lenses86 transmit four separate beams 80A-D from four spatially separatedlocations toward the mirror 1090. As shown in FIG. 12, the mirror 1090reflects each of the four beams 80A-D to a respective spatially distinctregion 1202A-D of the image field 1094.

Thus, the four beams 80A-D each illuminate four separate “tiles” 1202A-Dthat together form an entire image. One skilled in the art willrecognize that more than four tiles may form the image. For example,adding a third set of fiber/lens pairs could produce a 2-by-3 tile imageor a 3-by-2 tile image.

To produce an actual image, the intensity and color content of each ofthe beams 80A-D is modulated with image information as the mirror 1090sweeps through a periodic pattern, such as a raster pattern. FIG. 13shows diagrammatically one embodiment where the beams 80A-D can bemodulated in response to an image signal V_(IM) to produce the fourtiles 1202A-D.

The image signal V_(IM) drives an A/D converter 1302 that producescorresponding data to drive a demultiplexer 1304. In response to thedata and a clock signal CK from the controller 74 (FIG. 8), thedemultiplexer 1304 produces four output data streams, where each datastream includes data corresponding to a respective image tile 1202A-D.For example, the demultiplexer 1304 outputs data corresponding to thefirst half of the first line of the image to a first buffer 1306A andthe data corresponding to the second half of the first line to a secondbuffer 1306B. The demultiplexer 1304 then outputs data corresponding tothe second line of the image to the second lines of the first twobuffers 1306A, B. After the first two buffers 1306A, B contain datarepresenting the upper half of the image, the demultiplexer 1304 thenbegins filling third and fourth buffers 1306C, D. Once all of thebuffers 1306A-D are full, an output clock CKOUT clocks datasimultaneously from all of the buffers 1306A-D to respective D/Aconverters 1308A-D. The D/A converters 1308A-D then drive respectivelight sources 78 to produce light that is scanned into the respectiveregions 2102A-D, as described above. The actual timing of the pixeloutput is controlled by the output clock CKOUT, as described below withreference to FIGS. 28-31.

One skilled in the art will recognize that, although the system of FIG.13 is described for four separate regions 1201A-D, a larger or smallernumber of regions may be used. Also, where some overlap of the regions1202A-D is desired, common data can be stored in more than one buffer1202A-D. Because the sets of common data will duplicate some pixels inthe overlapping region, the data may be scaled to limit the intensity tothe desired level.

One approach to improving image quality that is helpful in “matching”the image portions 1202A-D to each other will now be described withreference to FIGS. 14 and 15. Because the angle of the beams 80A-D isdetermined by the angles of the vertical and horizontal scanner (for theuniaxial, two scanner system) or the horizontal and vertical angles ofthe single mirror (for the biaxial scanner), the actual vector angle ofthe beams 80A-D at any point in time can then be determined by vectoraddition. In most cases, the desired vertical portions of the scanpatterns will be a “stair step” scan pattern, as shown by the brokenline in FIG. 14.

If the turning mirror 100 (FIG. 8) is disabled, the pattern traced bythe ray will be the same as that described above with respect to FIGS.3-5. As represented by the solid line in FIG. 14, the actual verticalscan portion of the pattern, shown in solid line, will be an approximateramp, rather than the desired stair step pattern.

On approach to providing the stair step pattern would be to drive thevertical scanner 58 with the stair step voltage. However, because thevertical mirror is a physical system and the stair step involvesdiscontinuous motion, the vertical mirror will not follow the drivesignal exactly. Instead, as the vertical mirror attempts to follow thestair step pattern, the vertical mirror will move at a maximum rateindicated largely by the size and weight of the vertical mirror, thematerial properties of the mirror support structure, the peak voltage orcurrent of the driving signal, and electrical properties of the drivingcircuitry. For typical vertical scan mirror size, configuration, scanangle and driving voltage, the vertical scanner 58 is limited tofrequencies on the order of 100 to 3000 Hz. The desired scan pattern hasfrequency components far exceeding this range. Consequently, driving thevertical scanner 58 with a stair step driving signal can produce avertical scan pattern that deviates significantly from the desiredpattern.

To reduce this problem, the scanning assembly 82 of FIG. 8 separates thevertical scan function into two parts. The overall vertical scan is thena combination of a large amplitude ramp function at about 60 Hz and asmall amplitude correction function at twice the horizontal rate (e.g.,about 30 kHz). The vertical scanner 58 can produce the large amplituderamp function, because the 60 Hz frequency is well below the upperfrequency limit of typical scanning mirrors. Correction mirrors 100replace the turning mirrors 100 and provide the small amplitudecorrections. The correction mirrors 100 operate at a much higherfrequency than the vertical scanner; however, the overall angular swingsof the correction mirrors 100 are very small.

As can be seen from the signal timing diagram of FIG. 15, the correctionmirror 100 travels from approximately its maximum negative angle to itsmaximum positive angle during the time that the horizontal scanner scansfrom the one edge of the field of view to the opposite edge (i.e. fromtime t₁ to t₂ in FIG. 15). The overall correction angle, as shown inFIGS. 14 and 15, is defined by the amount of downward travel of thevertical scan mirror during a single horizontal scan. The correctionangle will vary for various configurations of the display; however, thecorrection angle can be calculated easily.

For example, for a display where each image region 1202A-D has 1280vertical lines and a total mechanical vertical scan angle of 10 degrees,the angular scan range for each line is about 0.008 degrees(10/1280=0.0078125). Assuming the vertical scanner 58 travels thisentire distance during the horizontal scan, an error correction to besupplied by the correction mirror 100 is about plus or minus 0.0039degrees. The angular correction is thus approximately θ/N, where θ isthe vertical scan angle and N is the number of horizontal lines. Thisnumber may be modified in some embodiments. For example, where thehorizontal scanner 56 is a resonant scanner, the correction angle may beslightly different, because the horizontal scanner 56 will use someportion of the scan time to halt and begin travel in the reversedirection, as the scan reaches the edge of the field of view. Thecorrection angle may also be modified to correct for aberrations inoptical elements or optical path length differences. Moreover, thefrequency of the correction scanner 100 may be reduced by half if datais provided only during one half of the horizontal scanner period(“unidirectional scanning”), although raster pinch is typically notproblematic in unidirectional scanning approaches.

As can be seen from the timing diagrams of FIGS. 14 and 15, thecorrection mirror 100 will translate the beam vertically by about onehalf of one line width at a frequency of twice that of the horizontalscanner 56. For a typical display at SVGA image quality withbi-directional scanning (i.e., data output on both the forward andreverse sweeps of the horizontal scanner 56), the horizontal scanner 56will resonate at about 15 kHz. Thus, for a typical display, thecorrection scanner 100 will pivot by about one-tenth of one degree atabout 30 kHz. One skilled in the art will recognize that, as theresolution of the display increases, the scan rate of the horizontalscanner 56 increases. The scan rate of the correction mirror 100 willincrease accordingly; but, the pivot angle will decrease. For example,for a display having 2560 lines and an overall scan of 10 degrees, thescan rate of the correction mirror 100 will be about 60 kHz with a pivotangle of about 0.002 degrees. One skilled in the art will recognizethat, for higher resolution, the minimum correction mirror size willtypically increase where the spot size is diffraction limited.

FIG. 16 shows a piezoelectric scanner 110 suitable for the correctionmirror 100 in some embodiments. The scanner 110 is formed from aplatform 112 that carries a pair of spaced-apart piezoelectric actuators114, 116. The correction mirror 100 is a metallized, substantiallyplanar silicon substrate that extends between the actuators 114, 116.The opposite sides of the piezoelectric actuators 114, 116 areconductively coated and coupled to a drive amplifier 120 such that thevoltage across the actuators 114, 116 are opposite. As is known,piezoelectric materials deform in the presence of electric fields.Consequently, when the drive amplifier 120 outputs a voltage, theactuators 114, 116 apply forces in opposite directions to the correctionmirror 100, thereby causing the correction mirror 100 to pivot. Oneskilled in the art will recognize that, although the piezoelectricactuators 114, 116 are presented as having a single set of electrodesand a single layer of piezoelectric material, the actuators 114, 116would typically be formed from several layers. Such structures are usedin commercially available piezoelectric devices to produce relativelylarge deformations.

A simple signal generator circuit 122, such as a conventional rampgenerator circuit, provides the driving signal for the drive amplifier120 in response to the detected position of the horizontal scanner 56.The principal input to the circuit 122 is a sense signal from a sensorcoupled to the horizontal scanner 56. The sense signal can be obtainedin a variety of approaches. For example, as described in U.S. Pat. No.5,648,618 to Neukermanns et al., entitled MICROMACHINED HINGE HAVING ANINTEGRAL TORSIONAL SENSOR, which is incorporated herein by reference,torsional movement of a MEMs scanner can produce electrical outputscorresponding to the position of the scanning mirror. Alternatively, theposition of the mirror may be obtained by mounting piezoelectric sensorsto the scanner, as described in U.S. Pat. No. 5,694,237 to Melville,entitled POSITION DETECTION OF MECHANICAL RESONANT SCANNER MIRROR, whichis incorporated herein by reference. In other alternatives, the positionof the beam can be determined by optically or electrically monitoringthe position of the horizontal or vertical scanning mirrors or bymonitoring current induced in the mirror drive coils.

When the sense signal indicates that the horizontal scanner 56 is at theedge of the field of view, the circuit 122 generates a ramp signal thatbegins at its negative maximum and reaches its zero crossing point whenthe horizontal scanner reaches the middle of the field of view. The rampsignal then reaches its maximum value when the horizontal scan reachesthe opposite edge of the field of view. The ramp signal returns to itsnegative maximum during the interval when the horizontal scan slows to ahalt and begins to return sweep. Because the circuit 122 can use thesense signal as the basic clock signal for the ramp signal, timing ofthe ramp signal is inherently synchronized to the horizontal position ofthe scan. However, one skilled in the art will recognize that, for someembodiments, a controlled phase shift of the ramp signal relative to thesense signal may optimize performance. Where the correction mirror 100is scanned resonantly, as described below with reference to FIG. 18, theramp signal can be replaced by a sinusoidal signal, that can be obtainedsimply be frequency doubling, amplifying and phase shifting the sensesignal.

The vertical movements of the beams 80A-D induced by the correctionmirrors 100 offset the movement of the beams 80A-D caused by thevertical scanner 58, so that the beams 80A-D remain stationary along thevertical axis during the horizontal scan. During the time the horizontalscan is out of the field of view, the beams 80A-D travel vertically inresponse to the correction mirrors 100 to the nominal positions of thenext horizontal scan.

As can be seen from the above discussion, the addition of thepiezoelectrically driven correction mirrors 100 can reduce the rasterpinching significantly with a ramp-type of motion. However, in someapplications, it may be undesirable to utilize ramp-type motion. Onealternative embodiment of a scanner 130 that can be used for thecorrection mirror 100 is shown in FIGS. 17A and 17B.

The scanner 130 is a resonant micorelectromechanical (MEMs) scanner,fabricated similarly to the uniaxial embodiment described in theNeukermanns '790 patent. Alternatively, the scanner 130 can be amechanically resonant scanner very similar to the horizontal scanner 54of FIG. 9; however, in such a scanner it is preferred that thedimensions and material properties of the plate and mirror be selectedto produce resonance at about 30 kHz, which is twice the resonantfrequency of the horizontal scanner 200. Further, the materials andmounting are preferably selected so that the scanner 130 has a lower Qthan the Q of the horizontal scanner 56. The lower Q allows the scanner130 to operate over a broader range of frequencies, so that the scanner130 can be tuned to an integral multiple of the horizontal scanfrequency.

The use of the resonant scanner 130 can reduce the complexity of theelectrical components for driving the scanner 130 and can improve thescanning efficiency relative to previously described approaches.Resonant scanners tend to have a sinusoidal motion, rather than thedesired ramp-type motion described above. However, if the frequency,phase, and amplitude of the sinusoidal motion are selectedappropriately, the correction mirror 100 can reduce the pinch errorsignificantly. For example, FIG. 18 shows correction of the rastersignal with a sinusoidal motion of the correction mirror where thehorizontal field of view encompasses 90 percent of the overallhorizontal scan angle. One skilled in the art will recognize that theerror in position of the beam can be reduced further if the field ofview is a smaller percentage of the overall horizontal scan angle.Moreover, even further reductions in the scan error can be realized byadding a second correction mirror in the beam path, although this isgenerally undesirable due to the limited improvement versus cost.Another approach to reducing the error is to add one or more higherorder harmonics to the scanner drive signal so that the scanning patternof the resonant correction scanner 130 shifts from a sinusoidal scancloser to a sawtooth wave.

In addition to correcting for raster pinch, the system of FIG. 8 alsoincludes the deformable mirror 180 that operates as a dynamic aberrationcompensation element. The deformable mirror 180 modifies the lens phasefunction on-the-fly, optimizing the lens for each pixel as it isaddressed during the scan. For this type of dynamic correction, thedeformable mirror 180 can change its shape rapidly, typically at twicethe line scan frequency, on the order of 40 kHz for the case of SVGAclass video devices. As will be described below with reference to FIGS.49-50 d, the deformable membrane 180 is fabricated using MEMStechnology.

The aberration correction does not necessarily require wavefrontsensing, as it is typically a deterministic function. Instead, thecontrolled deformable-membrane shape can be varied on a rapid time scaleunder electronic control to perform the correction function in thisembodiment.

In principle, a single micromachined adaptive mirror capable ofarbitrary surface figure could be used for dynamic correction in ascanned beam instrument, provided the mechanical bandwidth was highenough and the control scheme fast enough. Relevant approaches aredescribed in P. K. C. Wang, F. Y. Hadaegh, “Modal noninteractingcontrols for deformable mirrors,” Proc. IEEE Conference on ControlApplications v. 1, pp 121-128, 1993; and G. Vdovin, P. M. Sarro, S.Middelhoek, “Technology and applications of micromachined membranedeformable mirrors,” LEOS Summer Topical Meeting July 20-24, pp 97-98,1998, each of which is incorporated herein by reference.

Such a mirror would employ a high density interconnection between thecontroller and the actuator array, and the overall system would adopt aconsiderable degree of complexity. The approach described with respectto this embodiment corrects the primary aberrations with simple membranesurfaces actuated with a single electrode. Such surfaces are readilyfabricated, can be small and therefore high speed, and the controlinterface requires only a single connection for each surface. Dependingon the defects in the imaging system, correction may require reflectionfrom several specifically designed membranes.

Before describing the structure of the deformable membrane 180, thefollowing sections review the relevant aberration background and theory.The corresponding deformable membranes for this applications arereferred to herein as aberration compensation membranes (ACM).

Wavefront aberration, as used herein refers to deviations of the actualconstant phase wavefront produced by the optical system from a perfectsphere centered on the image point, as illustrated in FIG. 44. Acorrective element 4402, which may be an ACM, adds a variable opticalpath delay to the wavefront in order to just cancel the aberration andproduce a wavefront that is more closely approaches a sphericalwavefront. A reflective element 4404, such as a mirror, with surfacefigure s(x,y) will add a correction w(x,y)=2s(x,y).

For the purposes of numerical computation it is useful to specify aparticular plane for observation of the wavefront, which is typicallythe exit pupil of the system. FIG. 45 illustrates the coordinate systemused herein to describe the wavefront aberrations. The primary, orSeidel, aberrations are frequently given for the axially symmetricsystem as:w(x,y)=a _(s) r ⁴ +a _(c) r ³ cos(θ)h′+a _(a) r ² cos²(θ)h′ ² +a _(d) r² h′ ² +a _(t) r cos(θ)h′ ³

This calculation is described in greater detail in Virendra N. Mahajan,Aberration Theory Made Simple, SPIE press, Bellingham, Wash., 1991.Chapter 1, which is incorporated herein by reference.

In this equation, the five terms represent spherical, coma, astigmatism,field curvature (or defocus) and distortion (or tilt). It is assumedthat the image height h′ is on the x′ axis of the image plane, and thepupil plane coordinates are chosen so that the x and x′ axes areparallel. In a physical system, the pupil plane coordinates will befixed, and the image point (x′,y′) is not restricted to be on the x′axis, so we use a more general form for the aberration expression:w(x,y)=a _(s) r ⁴ +a _(c) r ²(xx′+yy′)+a _(a)(xx′+yy′)² +a _(d) r ² r′ ²+a _(t)(xx′+yy′)r′ ²

where r²=(x²+y²) and r′²=(x′²+y′²), and the coefficients are the same asfor the previous equation.

We can now consider each aberration term and the shape of a correctiveelement used to compensate.

Spherical: a_(s)r⁴

Referring now to FIG. 46, spherical aberration is a defect in thewavefront varying as r⁴, and it is independent of the location of theimage point in the field of view. Therefore correction of sphericalaberration can be accomplished with a static reflective or refractivesurface.

Coma: a_(c)r²(xx′+yy′)

Examining the second term in the aberration expansion we see that twosurfaces will be required to correct for coma associated with anarbitrary image point (x′,y′), one with surface figure s(x,y)=r²x withmagnitude ½ a_(c)x′, and the other with s(x,y)=r²y with magnitude ½a_(c)y′. These surfaces do not have a boundary that remains stationary,however, which is a requirement for an edge supported membrane. Notethat for a circular boundary with r constant, s(x,y) varies linearlywith either x or y. To correct for coma using an edge supported membranewe can use a modified corrective surface that includes an offsetingwavefront tilt, according to

${s_{cx}\left( {x,y} \right)} = {\frac{1}{2}\left( {a_{c}x^{\prime}} \right)\left( {{r^{2}x} - x} \right)}$${s_{cy}\left( {x,y} \right)} = {\frac{1}{2}\left( {a_{c}y^{\prime}} \right){\left( {{r^{2}y} - y} \right).}}$

This shape is illustrated in FIG. 46, and the ACM surface s_(cy) is thesame as s_(cx), but rotated by 90°. The tilt introduced is an additionaldistortion term, and can be compensated by the beam scan angle.

Astigmatism and Field Curvature: a_(a)(xx′+yy′)²+a_(d)r²r′²

Astigmatism and field curvature are aberrations of second order in thepupil plane coordinates, and ACM surfaces to compensate astigmatism andfield curvature behave like variable focus mirrors—that is, mirrors forwhich the curvature changes depending on the location of the imagepoint. Field curvature is corrected with an axially symmetric variablefocus lens, while astigmatism is corrected with a variable focuscylindrical lens. Expanding these expressions and collecting like termsin the pupil plane coordinates leads to:w _(astigmatism+field curvature)=(a _(a) x′ ² +a _(d)(x′ ² +y′ ²))x ²+2a_(a) x′y′xy+(a _(a) y′ ² +a _(d)(x′ ² +y′ ²))y ².

These three terms indicate a minimum of three surfaces to compensate.One strategy is to use three cylindrical surfaces that vary as x², y²,and (x cos(α)+y sin(α))² (which is equivalent to a rotation of the x²surface by an angle α). The appropriate membrane shape for these ACMelements is shown in FIG. 3 labeled Astigmatism. Recognizing thatx²+y²=r², another strategy would be to use a parabolic surface varyingas r², and two cylindrical surfaces varying as x² and (x cos(α)+ysin(α))². The parabolic surface can then be used as a general focuscontrol element in addition to controlling field curvature. Letting α=45°, it can be shown that the required ACM surfaces are specified by:

$\begin{matrix}\left. \begin{matrix}\begin{matrix}{{s_{ax}\left( {x,y} \right)} = {{\frac{1}{2}\left\lbrack {{a_{a}{x^{\prime}\left( {x^{\prime} - y^{\prime}} \right)}} + {a_{d}\left( {x^{\prime 2} + y^{\prime 2}} \right)}} \right\rbrack}x^{2}}} \\{{s_{a\; 45{^\circ}}\left( {x,y} \right)} = {a_{a}x^{\prime}{y^{\prime}\left( {\frac{x}{\sqrt{2}} + \frac{y}{\sqrt{2}}} \right)}^{2}}}\end{matrix} \\{{s_{ay}\left( {x,y} \right)} = {{\frac{1}{2}\left\lbrack {{a_{a}{y^{\prime}\left( {y^{\prime} - x^{\prime}} \right)}} + {a_{d}\left( {x^{\prime 2} + y^{\prime 2}} \right)}} \right\rbrack}y^{2}}}\end{matrix} \right\} & {{Approach}\mspace{20mu} 1}\end{matrix}$ or $\begin{matrix}\left. \begin{matrix}\begin{matrix}{{s_{ax}\left( {x,y} \right)} = {{\frac{1}{2}\left\lbrack {a_{a}\left( {x^{\prime 2} - y^{\prime 2}} \right)} \right\rbrack}x^{2}}} \\{{s_{a\; 45{^\circ}}\left( {x,y} \right)} = {a_{a}x^{\prime}{y^{\prime}\left( {\frac{x}{\sqrt{2}} + \frac{y}{\sqrt{2}}} \right)}^{2}}}\end{matrix} \\{{s_{ar}\left( {x,y} \right)} = {{\frac{1}{2}\left\lbrack {{a_{a}{y^{\prime}\left( {y^{\prime} - x^{\prime}} \right)}} + {a_{d}\left( {x^{\prime 2} + y^{\prime 2}} \right)}} \right\rbrack}r^{2}}}\end{matrix} \right\} & {{Approach}\mspace{20mu} 2}\end{matrix}$ Distortion  :  a_(t)(x x^(′) + y y^(′))r^(′2)

Distortion, or wavefront tilt, is a wave aberration linear in the pupilplane coordinates. The defect in the image amounts to a positioningerror of the image point, and correction for a scanned beam instrumentwould fall most naturally to the beam scanner or to a correction in thenumerical scan conversion performed by the system.

Therefore, excluding spherical aberration, which is independent of theimage point location, and distortion, which is correctable using thescanning mechanism, coma, astigmatism and field curvature are still tobe corrected on-the-fly. This may require as many as 5 separate surfacesfor correction at any point in the field-of-view.

Practical optical systems may be well corrected for one or the other ofthese defects, reducing the required number of ACM surfaces further.

One embodiment of an ACM surface for system aberration correctionmembrane 4700 uses the basic device geometry illustrated in FIG. 47. Themembrane consists of layers of poly1 4702, poly2 4704 and chrome-goldlayer 4706. The gold and chrome can be removed with a wet etch ifdesired.

The support for the membranes in this embodiment is a perimeter ring ofpoly1, approximately 2 μm thick and 10 μm wide. The ACM device 4700 canhave a range of dimensions which may depend upon the desired membranesupport stiffness and may have continuous support rings or segmentedsupports. In one exemplary embodiment, the weakest supports had a 10%duty width for the polysilicon flexure.

Clearance beneath the membranes is established in this embodiment byoxide1 4708 which is 2 μm thick. Dimples 4710 help reduce stiction andproduce a clearance of approximately 1.25 μm. A drive electrode 4712 forthe membranes is constructed from poly0. In this embodiment, theelectrode 4712 was continuous with an identical footprint to the movablemembrane. One skilled in the art will recognize that other electrodestructures, such as segmented electrodes may be desired and may providemore precise control of deformation of the membrane.

The membranes are circular, and for this embodiment, range in diameterfrom 100 μm to 500 μm. However, other dimensions may be useful in someapplications. For example where the beam width is on the order of 1-2mm, the deformable membrane may be of similar or larger dimensions. Anexample of one such membrane 4800 is shown in the interferencephotomicrograph of FIG. 48 with a 200 μm diameter mirror. Thechrome-gold layer is left intact in this embodiment. Because thephotomicrograph is taken at normal incidence with 633 nm wavelengthillumination, each full fringe represents 316 nm surface heightdifferential.

Deformation proportional to x² as produced by a rectangular membrane4900 is used for astigmatism control in the embodiment of FIG. 49. Therectangular membrane 4900 is supported on two opposite sides. Becauseintrinsic stress gradients in the composite membranes will cause curlingat the free edges of the membrane 4900, the devices have a 2-to-1width-to-length ratio. The beam in this embodiment will typicallyilluminate a circular region near the center of the membrane. Theinterference photomicrograph FIG. 49 taken at normal incidence with 633nm wavelength illumination shows that the rectangular membrane 4900 hasa 250 μm×500 μm rectangular mirror, with the chrome-gold layer intact.

To evaluate the devices, an optical imaging interferometer can measurethe surfaces of the mirrors. For evaluation, it may be desirable to tiltthe mirrors with respect to the image beam to create several fringesacross the aperture, typically more than 20. Image processing can thenextract fringe locations and indicate local surface height. Most of thedata presented here indicate cross-sectional surface height, taken froma section through the center of circular membranes, and through thecenter and parallel to the free edges of the rectangular membranes.Polynomial curve fits can assess surface figure accuracy of the measuredsurface height to determine r² and r⁴ coefficients for the circularmembranes and x² and x⁴ coefficients for the rectangular membranes.

FIG. 50 a shows surface cross-sections of a 200 μm diameter circularmirror with a continuous poly1 support ring, for various controlvoltages. FIG. 50 b shows similar cross-sections for a 250 μm×500 μmrectangular mirror. The tensile stress in the chrome-gold layer causesan initial concave deflection in the presence of zero electrode voltage.The displacement is more severe for larger devices—the 200 μm circle hasapproximately 0.25 μm deflection at its center, while the 250 μm longrectangle has over 0.5 μm bias deflection. Greater range of motion canbe achieved by etching the metal layer, in which case the zero voltageshape is slightly convex. FIGS. 50 c and 50 d show the devices of FIGS.50 a and 50 b, respectively, without the metal layer.

If the full physical clearance were used, these membranes would deflectup to 1.25 μm deflection, which corresponds to 4 waves of aberrationwhen reflecting 633 μm illumination. However, for this structure pull-ineffects can limit the useful free range. Such displacements can be onthe order of 0.4 to 0.5 μm stable DC membrane displacements. For thisstructure, this can limit the number of waves of aberration compensationto about 1.5. However, other geometries an structures may no be solimited. For larger devices with chrome-gold coating, the actuationrange may be limited by stress curvature.

As shown the surface profile in FIG. 51 (where control voltage is 0V and55V), a rectangular membrane gold etch shows almost no edge curling.Such a surface plot can be constructed from line scans taken from aninterference image of a released device. The end view shows theuniformity across the entire membrane width.

A membrane surface that varies as r² can correct defocus and curvatureof field. Near the supported perimeter of the membrane, the shapedeviation from this prescription may be less significant.

The next higher term in a polynomial fit to the surface shape varies r⁴,which is a spherical aberration. Spherical aberration introduced for agiven amount of focus correction can indicate relative performance ofdifferent membrane shapes and limits of the useable aperture of themembrane mirror.

For astigmatism compensation using the rectangular mirrors, the desiredsurface is proportional to x², and the next expansion term is x⁴. Theterm “spherical aberration” as used herein describes the fourth orderterms for both the circular and rectangular membranes, although oneskilled in the art will recognize that this is a simplification of thedefect for clarity of presentation.

In the described embodiments, stiffer supports typically introduce morespherical aberration for a given amount of defocus than the weakersupports, enabling the use of a greater portion of the membrane by theoptical beam. However, where the membranes are larger than 200 μm indiameter, the improvement may not be as significant. FIG. 52 shows curvefits for a 200 μm diameter circular polysilicon membrane, along with theresidual spherical aberration of the surface, for the cases of acontinuous support ring and a 10% duty width ring, both with and withoutChrome-Gold on the top surface.

A figure of merit that is useful in evaluating the spherical aberrationfor a particular deflection is: a₄/a₂ ², where a₂ and a₄ are the secondorder and fourth order polynomial coefficients for the curve fit.Physically the quantity a₄/a₂ ² is the spherical aberration, in μm, atthe radius where the quadratic deflection is 1 μm. While this may or maynot be a displacement that is physically realized by the membrane, thefigure of merit allows comparison of the relative amount of sphericalaberration for the various configurations. For the case of gold coatedmembranes, the spherical aberration with the weaker support (10% dutywidth) of this embodiment is a factor of two smaller than for the fullstiffness support, and for the membrane without gold the improvement isa factor of four. While the figure of merit is somewhat arbitrary, itcan be useful in evaluating relative performance. A specification ofspherical aberration less than λ/10 would provide a useable aperture ofabout 65% of the mirror with less than 1 wave of focus control. A 10%support may achieve less than λ/10 spherical aberration oversubstantially the entire 200 μm aperture with 1.5 waves of focus controlfor the circular embodiment.

The 10% stiffness support embodiment can therefore correct more than 1.5waves of defocus/field curvature at 633 nm while introducing less thanλ/10 spherical aberration using the 200 μm diameter device. Similarlythe 250 μm×500 μm rectangular membrane with 25% stiffness supports cancorrect more than 1 wave of astigmatism while introducing less than λ/10fourth order aberration.

For dynamic correction during beam scanning, the ACM changes shape asthe beam is scanning, with a full deflection bandwidth typically greaterthan approximately twice the line scan frequency. This may be on theorder of 10-100 kHz, though the frequency response may be limited by avariety of factors, including size and stiffness.

As shown in FIG. 53, a test set up 5300 utilizes an ACM with a 300 μmcircular membrane 5302 with 25% support strength, as well as an ACM witha 250 μm×500 μm rectangular membrane 5304 and a 25% support strength forcorrecting astigmatism and field curvature in a one dimensional beamscanner 5310. The scanner 5310 includes a 100 mm focal length achromaticdoublet as the scan lens, with a scan mirror 5314 placed 20 mm behindthe lens. This configuration has negligible spherical aberration andcoma, but it has astigmatism coefficient a_(a)=4.7E−4 cm⁻³ and fieldcurvature coefficient a_(d)=1.8E−4 cm⁻³, so that scanning to a beamangle of 7.5° introduces 1 wave of astigmatism and 0.4 wave of defocus.The circular and rectangular ACM elements 5302, 5304 correct theastigmatism and field curvature. Two telescopes 5316, 5318 demagnify the1 mm diameter beam from a HeNe laser to just fill the aperture of thetwo ACM devices 5302, 5304. A third telescope 5320 expands the beam to5.6 mm before it hits the scan mirror. The telescopes 5316, 5318, 5320also image the two ACM surfaces onto the scan mirror 5314, whichoperates as the aperture stop in the system.

Image 5402 in FIG. 54 shows an image of the focused spot at the centerof the field of view, with no correction, and image 5404 shows the spotat full field scan angle with no correction. Image 5406 shows the spotwith focus correction and image 5408 shows the spot with both focus andastigmatism correction. The ACM devices compensate the aberrationsintroduced by scanning in this one dimensional demonstration to producean increased field of view that more closely approaches a diffractionlimited field of view.

The ACMs described herein are exemplary and are not intended to limitthe scope of the invention. ACM devices may be capable of largerdeflections and greater aberration correction using different materials,support structures and electrostatic deformation forces.

Moreover the application is not limited to a resonant scanning approach,or even a scanning system. For example, the aberration correctiondescribed here can be applied to a scanning system using a spinningpolygon or other type of scanner. Moreover, the deformable mirrorcorrections can also be used for optical coupling in a switched fibersystem that employs mirrors to selectively direct light from one fiberto another or to printing systems where a mirror directs light from anoptical source toward a print medium.

The deformable membrane can be combined with a variety of the otherelements described herein for certin applications. For ecxample, asdescribed above, the deformable membrane may be carried by thecorrection mirror so that a single device can perform both raster pinchand aberration correction. Further, as described in the Furness patent,the deformable membrane may also provide a focus effect, although thefrequency of such activation may be considerably higher than foraberration correction. In another aspect of the invention, it is usefulto note that in many applications, the membrane will be driven at anintegral multiple of the scanning frequency. For example in a scannedbeam display the membrane would typically be actuated at twice thehorizontal scan frequency. Consequently, it may be useful in someapplications to synchronize the membrane motion to an integral of thescan frequency. For relatively simple corrections, the membrane couldthen be designed and operated as a resonant device.

Another alternative embodiment of a reduced error scanner 140 is shownin FIG. 19 where the scan correction is realized by adding a verticalcomponent to a horizontal mirror 141. In this embodiment, the horizontalscanner 140 is a MEMs scanner having an electrostatic drive to pivot thescan mirror. The horizontal scanner 140 includes an array of locations143 at which small masses 145 may be formed. The masses 145 may bedeposited metal or other material that is formed in a conventionalmanner, such as photolithography. Selected ones of the masses 143 areremoved to form an asymmetric distribution about a centerline 147 of themirror 141. The masses 145 provide a component to scan the correctionalong the vertical axis by pivoting about an axis orthogonal to itsprimary axis. As can be seen in FIG. 20, the vertical scan frequency isdouble the horizontal scan frequency, thereby producing the Lissajous or“bow-tie” overall scan pattern of FIG. 20. The masses 145 may beactively varied (e.g. by laser ablation) to tune the resonant frequencyof the vertical component. This embodiment allows correction without anadditional mirror, but typically requires matching the resonantfrequencies of the vibration and the horizontal scanner.

To maintain matching of the relative resonant frequencies of thehorizontal scanner 56 and the correction scanner 100, the resonantfrequency of either or both scanners 56, 100 may be tuned actively.Various frequency control techniques are described below with referenceto FIGS. 33-36. Where the Q of the scanners 56, 100 are sufficiently lowor where the scanners 56, 100 are not resonant, simply varying thedriving frequency may shift the scanning frequency sufficiently tomaintain synchronization.

As shown in FIG. 21, another embodiment of a scanner 150 according tothe invention employs a biaxial scanner 152 as the principal scancomponent, along with a correction scanner 154. The biaxial scanner 152is a single mirror device that oscillates about two orthogonal axes.Design, fabrication and operation of such scanners are described forexample in the Neukermanns '790 patent, in Asada, et al, SiliconMicromachined Two-Dimensional Galvano Optical Scanner, IEEE Transactionson Magnetics, Vol. 30, No. 6, 4647-4649, November 1994, and in Kiang etal, Micromachined Microscanners for Optical Scanning, SPIE proceedingson Miniaturized Systems with Micro-Optics and Micromachines II, Vol.3008, February 1997, pp. 82-90 each of which is incorporated herein byreference. The bi-axial scanner 152 includes integral sensors 156 thatprovide electrical feedback of the mirror position to terminals 158, asis described in the Neukermanns '618 patent.

The correction scanner 154 is preferably a MEMs scanner such as thatdescribed above with reference to FIGS. 17A-B, although other types ofscanners, such as piezoelectric scanners may also be within the scope ofthe invention. As described above, the correction scanner 154 can scansinusoidally to remove a significant portion of the scan error; or, thecorrection mirror can scan in a ramp pattern for more precise errorcorrection.

Light from the light source 78 strikes the correction scanner 154 and isdeflected by a correction angle as described above. The light thenstrikes the biaxial scanner 152 and is scanned horizontally andvertically to approximate a raster pattern, as described above withreference to FIGS. 3-5.

Another embodiment of a display according to the invention, shown inFIG. 23, eliminates the correction mirror 100 by physically shifting theinput beam laterally relative to the input of an optical system 500. Inthe embodiment of FIG. 23, a piezoelectric driver 502 positioned betweena frame 504 and an input fiber 506 receives a drive voltage at afrequency twice that of the horizontal scan frequency. Responsive to thedrive voltage, the piezoelectric driver 502 deforms. Because the fiber506 is bonded to the piezoelectric driver 502, deformation of thepiezoelectric driver 502 produces corresponding shifting of the fiber506 as indicated by the arrow 508 and shadowed fiber 510. One skilled inthe art will recognize that, depending upon the characteristics of theoptical system 500, the piezoelectric driver 502 may produce lateraltranslation of the fiber 506 or angular shifting of the fiber 506output. The optical system 500 then translates movement of the fiberoutput into movement of the perceived pixel location as in thepreviously described embodiments. While the embodiment of FIG. 23translates a fiber, the invention is not so limited. For example someapplications may incorporate translation of other sources, such as LEDsor laser diodes, may translate the position of the lens 50, or maytranslate or rotate an entire scanner, such as a biaxial MEMs scanner.

Although the embodiment of FIG. 23 shifts the input beam by shifting theposition of the input fiber, other methods of shifting the input beammay be within the scope of the invention. For example, as shown in FIG.24, an electro-optic crystal 300 shifts the input beam 83 in response toan electrical signal. In this embodiment, the beam 83 enters a firstface 302 of a trapezoidally shaped electro-optic crystal 300, whererefraction causes a shift in the direction of propagation. When the beam83 exits through a second face 304, refraction produces a second shiftin the direction of propagation. At each face, the amount of changes inthe direction or propagation will depend upon difference in index ofrefraction between the air and the crystal 300.

As is known, the index of refraction of electro-optical crystals isdependent upon the electric field through the crystal. A voltage appliedacross the crystal 300 through a pair of electrodes 306 can control theindex of refraction of the crystal 300. Thus, the applied voltage cancontrol the index of refraction of the crystal 300. Thus the appliedvoltage can control the angular shift of the beam 83 as it enters andexits the crystal 300 as indicated by the broken line 83 a. The amountof shift will correspond to the applied voltage. Accordingly, the amountof shift can be controlled by controlling the voltage applied to theelectrodes 306. The crystal 300 thus provides a voltage controlled beamshifter that can offset raster pinch.

Although the embodiments described herein have been displays, otherdevices or methods may be within the scope of the invention. Forexample, as shown in FIG. 24, an imager 600 includes a biaxial scanner602 and correction scanner 604 that are very similar to the scanners152, 154 of FIG. 21. The imager 600 is an image collecting device thatmay be the input element of a digital camera, bar code reader, twodimensional symbol reader, document scanner, or other image acquisitiondevice. To allow the imager 600 to gather light efficiently, the imager600 includes gathering optics 606 that collect and transmit light from atarget object 608 outside of the imager 600 onto the correction scanner604. The gathering optics 606 are configured to have a depth of field,focal length, field of view and other optical characteristicsappropriate for the particular application. For example, where theimager 600 is a two dimensional symbology reader, the gathering optics606 may be optimized for red or infrared light and the focal length maybe on the order of 10-50 cm. For reading symbols at a greater distance,the focusing optics may have longer focusing distance or may have avariable focus. The optics may be positioned at other locations alongthe optical path to allow smaller, cheaper components to be used.

The correction scanner 604 redirects light received from the gatheringoptics 606 as described above for the display embodiments, so that thegathered light has a correction component before it reaches the biaxialscanner 602. The biaxial scanner 602 scans through a generally rasterpattern to collect light arriving at the gathering optics 606 from arange of angles and to redirect the light onto a group of stationaryphotodetectors 610, each positioned at a respective location andorientation, such that it images a respective “tile” of the image field.

Movement of the biaxial scanner 602 thus translates to imagingsuccessive points of the target object 608 onto the photodetectors 610.The photodetectors 610 convert light energy from the scanner 602 intoelectrical signals that are received by decoding electronics 612. Wherethe imager 600 is a symbology reader, the decoding electronics 612 mayinclude symbol decoding and storing circuitry and further electronicsfor assembling the image form the stored files. Where the imager is aportion of a camera, the decoding electronics 612 may includedigital-to-analog converters, memory devices and associated electronicsfor storing a digital representation of the scanned tile and furtherelectronics for assembling the image from the stored files. One skilledin the art will recognize that, although the correction scanner 604 ispositioned before the bi-axial scanner 602, it may be desirable toposition the correction scanner 604 following the bi-axial scanner 602in some applications.

Another feature of the imager 600 shown in FIG. 24 is a set ofillumination sources 614 that provide light for illuminating respectivelocations on a target object. The illumination sources 614 arepreferably of different wavelengths to ease differentiation of beams,although in some applications common wavelength devices may be used. Inone example of a multiwavelength structure where imager 600 is a symbolreader, the illumination sources 614 may include infrared or red lightemitters that emit beams of light into a beam splitter 616. The beamsplitter 616 directs the illuminating light beams into the biaxialscanner 602 where the illuminating light is redirected to the correctionscanner 604. Because the illuminating light beams are collinear with thepaths of light from the target object 608, the illuminating light beamsstrike the target object 608 at the same locations that are imaged bythe photodetectors 610. The illuminating light beams are reflected bythe target object 608 in pattern corresponding to the reflectivity ofthe respective regions of the target object 608. The reflectedilluminating light travels to the photodetectors 610 to image therespective regions light that can be used only by the photodetectors 610to image the respective regions of the target object 608. For highresolution, the area illuminated by the sources 614 or imaged by thephotodetectors 610 may be made small through a variety of known opticaltechniques. One skilled in the art will recognize that, although FIG. 24shows the correction scanner 604 positioned after the horizontal scanner602, it will often be preferable to position the correction scanner 604between the beam splitter 616 and the horizontal scanner 602. Thisallows for the mirror of the correction scanner 604 to be made small.

Alternatively, the photodetectors 610 may be mounted externally to thescanners 602, 604 and oriented to capture light directly from theirrespective tiles. Because each photodetector 610 is wavelength matchedto its respective source and because the photodetectors 610 are alignedto spatially distinct regions, crosstalk between signals from therespective tiles may be adequately suppressed.

In one application of the imager 600 of FIG. 24, one or more of theillumination sources 614 includes a visible, directly modulated lightsource, such as a red laser diode or a visible wavelength light emitteddiode (LED). As shown in FIG. 25, the visible illumination source 614can thus produce a visible image for the user. In the exemplaryembodiment of FIG. 25, the imager can operate as a symbology scanner toidentify information contained in a symbol on the target object 608.Once the decoding electronics 612 identifies a desired image to beviewed, such as an item price and identity, the decoding electronics 612modulates the drive current of the illumination sources 614 to modulatethe intensity of the emitted light according to the desired image. Whenthe user directs the imager 600 toward a screen 619 (or the targetobject), the illuminating light is scanned onto the screen 619 asdescribed above. Because the illuminating light is modulated accordingto the desired image, the visible light reflected from the screen 619 isspatially modulated according to the desired image. The imager 600 thusacts as an image projector in addition to acquiring image data. Inaddition to, or as an alternative to, modulating the diode to produce animage, the diodes corresponding to each of the regions of the targetobject 608 may also output continuous or pulsed beams of light that fillthe entire field of view of the imager 600. The imager 600 thus providesa spotter frame 618 that indicates the field of view to the user.Similarly, the illumination sources 614 can be modified to outline thefield of view or to produce other indicia of the field of view, such ascross hatching or fiducials, to aid the user in aligning the imager 600to the target object 608.

In addition to compensating for raster pinch, one embodiment of thescanning system, shown in FIG. 28, also addresses effects of thenonlinearity of resonant and other nonlinear scanning systems. Oneskilled in the art will recognize that, although this correction isdescribed for a single light source or single detector system, theapproaches described herein are applicable to systems using more thanone light source, as presented in FIG. 10 above. For example, in oneapplication, the corrected output clock signal described below withreference to FIG. 28, drives all of the buffers 1306A-D (FIG. 13) tooutput data in parallel from buffers 1306A-D.

As shown by broken line in FIG. 26, the timing of incoming data ispremised upon a linear scan rate. That is, for equally spaced subsequentlocations in a line, the data arrive at constant intervals. A resonantscanner, however, has a scan rate that varies sinusoidally, as indicatedby the solid line in FIG. 26. For a start of line beginning at time t₀(note that the actual start of scan for a sinusoidal scan would likelybe delayed slightly as described above with respect to FIG. 26), thesinusoidal scan initially lags the linear scan. Thus, if the image datafor position P₁ arrive at time t_(1A), the sinusoidal scan will placethe pixel at position P₂.

To place the pixel correctly, the system of FIG. 28 delays the imagedata until time t_(1B), as will now be described. As shown in FIG. 28,arriving image data V_(IM) are clocked into a line or frame buffer 2200by a counter circuit 2202 in response to a horizontal synchronizationcomponent of the image data signal. The counter circuit 2202 is aconventional type circuit, and provides an input clock signal havingequally spaced pulses to clock the data into the buffer 2200. In themultisource system of FIG. 13, the four buffers 1306A-D, anddemultiplexer 1304 replace the frame buffer and the image data areclocked sequentially through the demultiplexer 1304 into the fourbuffers 1306A-D, rather than being clocked into a single frame buffer orline buffer.

A feedback circuit 2204 controls timing of output from the buffer 2200(or buffers 1306A-D of FIG. 13). The feedback circuit 2204 receives asinusoidal or other sense signal from the scanning assembly 82 anddivides the period of the sense signal with a high-speed second counter2206. A logic circuit 2208 produces an output clock signal in responseto the counter output.

Unlike the input clock signal, however, pulses of the output clocksignal are not equally spaced. Instead, the pulse timing is determinedanalytically by comparing the timing of the linear signal of FIG. 26 tothe sinusoidal signal. For example, for a pixel to be located atposition P₁, the logic circuit 2208 provides an output pulse at timet_(1B), rather than time t_(1A), as would be the case for a linear scanrate.

The logic circuit 2208 identifies the count corresponding to a pixellocation by accessing a look-up table in a memory 2210. Data in thelook-up table 2210are defined by dividing the scanning system periodinto many counts and identifying the count corresponding to the properpixel location. FIG. 27 shows this evaluation graphically for a 35-pixelline. One skilled in the art will recognize that this example issimplified for clarity of presentation. A typical line may includehundreds or even thousands of pixels. As can be seen, the pixels will bespaced undesirably close together at the edges of the field of view andundesirably far apart at the center of the field of view. Consequently,the image will be compressed near the edges of the field of view andexpanded near the middle, thereby forming a distorted image.

As shown by the upper line, pixel location varies nonlinearly for pixelcounts equally spaced in time. Accordingly, the desired locations ofeach of the pixels, shown by the upper line, actually correspond tononlinearly spaced counts. For example, the first pixel in the upper andlower lines arrives at the zero count and should be located in the zerocount location. The second pixel arrives at the 100 count, but, shouldbe positioned at the 540 count location. Similarly, the third pixelarrives at count 200 and is output at count 720. One skilled in the artwill recognize that the figure is merely representative of the actualcalculation and timing. For example, some output counts will be higherthan their corresponding input counts and some counts will be lower. Ofcourse, a pixel will not actually be output before its correspondingdata arrives. To address this condition, the system of FIG. 28 actuallyimposes a latency on the output of data, in a similar fashion tosynchronous memory devices. For the example of FIG. 27, a single linelatency (3400 count latency) would be ample. With such a latency, thefirst output pixel would occur at count 3400 and the second would occurat count 3940.

FIG. 29 shows an alternative approach to placing the pixels in thedesired locations. This embodiment produces a corrected clock from apattern generator rather than a counter to control clocking of outputdata. A synch signal stripper 2500 strips the horizontal synchronizationsignal form an arriving image signal V_(IM). Responsive to the synchsignal, a phase locked loop 2502 produces a series of clock pulses thatare locked to the synch signal. An A/D converter 2504, driven by theclock pulses, samples the video portion of the image signal to producesampled input data. The sampling rate will depend upon the requiredresolution of the system. In the preferred embodiment, the sampling rateis approximately 40 Mhz. A programmable gate array 2506 conditions thedata from the A/D converter 2504 to produce a set of image data that arestored in a buffer 2508. One skilled in the art will recognize that, foreach horizontal synch signal, the buffer will receive one line of imagedata. For a 1480×1024 pixel display, the system will sample and store1480 sets of image data during a single period of the video signal.

Once each line of data is stored in the buffer 2508, the buffer isclocked to output the data to a RAMDAC 2509 that includes a gammacorrection memory 2510 containing corrected data. Instead of using thebuffer data as a data input to the gamma correction memory 2510, thebuffer data is used to produce addressing data to retrieve the correcteddata from the gamma correction memory 2510. For example, a set of imagedata corresponding to a selected image intensity I1 identifies acorresponding location in the gamma correction memory 2510. Rather thanoutput the actual image data, the gamma correction memory 2510 outputs aset of corrected data that will produce the proper light intensity atthe user's eye. The corrected data is determined analytically andempirically by characterizing the overall scanning system, including thetransmissivity of various components, the intensity versus currentresponse of the light source, diffractive and aperture effects of thecomponents and a variety of other system characteristics.

In one embodiment shown in FIG. 30 according to the invention, the datamay be corrected further for temperature-versus-intensity orage-versus-intensity variations of the light source. Reference datadrives the light source while the vertical and horizontal position isout of the user's field of view. For example, at the edge of thehorizontal scan, the reference data is set to a predetermined lightintensity. A detector 2519 monitors the power out of the light source2516 and a temperature compensation circuit 2521. If the intensity ishigher than the predetermined light intensity, a gain circuit 2523scales the signal from the RAMDAC 2506 by a correction factor that isless than one. If the intensity is higher than the predetermined lightintensity, the correction factor is greater than one. While theembodiments described herein pick off a portion of the unmodulated beamor sample the beam during non-display portions of the scanning period,the invention is not so limited. For example, a portion of the modulatedbeam can be picked off during the display portion of the scanning periodor continuously. The intensity of the picked off portion of themodulated beam is then scaled and compared to the input video signal todetermine shifts in the relative intensity of the displayed light versusthe desired level of the displayed light to monitor variations.

In addition to monitoring the intensity, the system can also compensatefor pattern dependent heating through the same correction data or bymultiplying by a second correction factor. For example, where thedisplayed pattern includes a large area of high light intensity, thelight source temperature will increase due to the extended period ofhigh level activation. Because data corresponding to the image signal isstored in a buffer, the data is available prior to the actual activationof the light source 2516. Accordingly, the system can “look-ahead” topredict the amount of heating produced by the pattern. For example, ifthe light source will be highly activated for the 50 pixels precedingthe target pixel, the system can predict an approximate patterndependent heat effect. The correction factor can then be calculatedbased upon the predicted pattern dependent heating. Although thecorrection has been described herein for the intensity generally, thecorrection in many embodiments can be applied independently for red,green and blue wavelengths to compensate for different responses of theemitters and for variations in pattern colors. Compensating for eachwavelength independently can help limit color imbalance due to differingvariations in the signal to intensity responses of the light emitters.

Returning to FIG. 29, the corrected data output from the gammacorrection memory 2510 (as it may be modified for intensity variations)drives a signal shaping circuit 2514 that amplifies and processes thecorrected analog signal to produce an input signal to a light source2516. In response, the light source 2516 outputs light modulatedaccording to the corrected data from the gamma correction memory 2510.The modulated light enters a scanner 2518 to produce scanned, modulatedlight for viewing.

The clock signal that drives the buffer 2508, correction memory 2510,and D/A converter 2512 comes from a corrected clock circuit 2520 thatincludes a clock generator 2522, pattern memory 2524 and rising edgedetector 2526. The clock generator 2522 includes a phase locked loop(PLL) that is locked to a sense signal from the scanner 2518. The PLLgenerates a high frequency clock signal at about 80 MHz that is lockedto the sense signal. The high frequency clock signal clocks datasequentially from addresses in the pattern memory 2524.

The rising edge detector 2526 outputs a pulse in response to each 0-to-1transition of the data retrieved from the pattern memory 2524. Thepulses then form the clock signal CKOUT that drives the buffer output,gamma correction memory 2510, and D/A converter 2512.

One skilled in the art will recognize that the timing of pulses outputfrom the edge detector 2526 will depend upon the data stored in thepattern memory 2524 and upon the scanning frequency f_(SCAN) of thescanner 2518. FIG. 31 shows a simplified example of the concept. Oneskilled in the art will recognize that, in FIG. 31, the data structureis simplified and addressing and other circuitry have also been omittedfor clarity of presentation.

In the example of FIG. 31, if the scanning frequency f_(SCAN) is 20 kHzand clock generator 2522 outputs a clock signal at 4000 times thescanning frequency f_(SCAN), the pattern memory 2524 is clocked at 80MHz. If all bits in an addressed memory location 2524A are 0, notransitions of the output clock occur for 16 transitions of thegenerator clock. For the data structure of lcoation 2524B, a singletransition of the output clock occurs for 16 transitions of thegenerator clock. Similarly, location 2524C provides two pulses of thegenerator clock in one period of the scan signal and location 2524Eprovides eight pulses of the generator clock in one period.

The number and relative timing of the pulses is thus controlled by thedata stored in the pattern memory 2524. The frequency of the generatorclock on the other hand depends upon the scanner frequency. As thescanner frequency varies, the timing of the pulses thus will vary, yetwill depend upon the stored data in the pattern memory.

The approaches of FIGS. 29 and 30 are not limited to sinusoidal ratevariation correction. The clock pattern memory 2524 can be programmed toaddress many other kinds of nonlinear effects, such as opticaldistortion, secondary harmonics, and response time idiosyncrasies of theelectronic and optical source.

Moreover, the basic structure of FIG. 29 can be modified easily to adaptfor vertical scanning errors or optical distortion, by inserting a bitcounter 2530, look up table 2532, and vertical incrementing circuit 2534before the buffer 2508, as shown in FIG. 30. The counter 2530 addressesthe look up table 2532 in response to each pulse of the input clock toretrieve two bits of stored data. The retrieved data indicate whetherthe vertical address should be incremented, decremented or leftunaffected. The data in the look up table 2532 is determined empiricallyby measuring optical distortion of the scanning system and optics or isdetermined analytically through modeling. If the address is to beincremented or decremented, the incrementing circuit increments ordecrements the address in the buffer 2508, so that data that was to bestored in a nominal memory location are actually stored in an alternatelocation that is one row higher or lower than the nominal location.

A graphical representation of one such data structure is shown in thesimplified example FIG. 32. In this example, the first three sets ofdata bits 3202 for the first line of data (line 0) are stored in thefirst memory row, the next three sets of data bits 3204 for the firstline are stored in the second memory row, and the last three sets ofdata bits are stored in the third memory row. One skilled in the artwill recognize that this example has been greatly simplified for clarityof presentation. An actual implementation would include many more setsof data and may utilize-decrementing of the row number as well asincrementing.

The result is that some portion of the data for one line is moved to anew line. The resulting data map in the buffer 2508 is thus distorted ascan be seen from FIG. 32. However, distortion of the data map can beselected to offset vertical distortion of the image caused by scanningand optical distortion. The result is that the overall system distortionis reduced. Although the embodiment of FIG. 30 shows correction ofvertical distortion by adjusting the position of data stored in thebuffer 2508, other approaches to this correction may be implemented. Forexample, rather than adjusting the addresses of the storage locations,the addresses used for retrieving data from the buffer 2508 to theRAMDAC 2509 can be modified.

As noted above, in many applications, it is desirable to control thescanning frequencies of one or more scanners. In non-resonant or low Qapplications, simply varying the frequency of the driving signal canvary the scanning frequency. However, in high Q resonant applications,the amplitude response of the scanners may drop off dramatically if thedriving signal differs from the resonant frequency of the scanner.Varying the amplitude of the driving signal can compensate somewhat, butthe magnitude of the driving signal may become unacceptably high in manycases. Consequently, it is undesirable in many applications to try tocontrol the scanner frequency f_(SCAN) simply by controlling the drivingsignal frequency and/or amplitude.

One approach to controlling the frequency f_(SCAN) is shown in FIGS. 33and 34 for a MEMs scanner 3300. The scanner 3300 includes four tuningtabs 3302A-D positioned at corners of a mirror body 3304. The tuningtabs 3302A-D are flexible projections that are integral to the mirrorbody 3304. Fixed rigid projections 3305 project from the mirror body3304 adjacent to the tuning tabs 3302A-D, leaving a small gaptherebetween.

Each of the tuning tabs 3302A-D carries a ground electrode 3306 coupledby a conductor 3310 to an external electrode 3312 to form an electricalreference plane adjacent to the respective tab 3302A-D. Each of therigid projections 3306A-D carries a respective hot electrode 3308controlled by a respective external electrode 3316A-D, that allowscontrol of the voltage difference between each tuning tab 3302A-D andits corresponding rigid projection 3306A-D.

Each flexible tab 3302A-D is dimensioned so that it bends in response toan applied voltage difference between the tab 3302A-D and the adjacentrigid projection 3306, as shown in FIG. 34. The amount of bending willdepend upon the applied voltage, thereby allowing electrical control oftuning tab bending.

One skilled in the art will recognize that the resonant frequency of thescanner 3300 will be a function of the mass of the mirror 3304, thedimensions and mechanical properties of torsion arms 3317 supporting themirror 3304, and the locations 3318 of the centers of mass of each halfof the mirror 3304 (including its tabs 3302A-D and rigid projections3306) relative to the axis of rotation of the mirror 3304. Bending theflexible tabs shifts the centers or mass slightly inwardly from theoriginal locations 3318 to new locations 3320. Because the centers ofmass are located closer to the axis of rotation, the scanning frequencyincreases slightly. Increasing the voltage on the fixed projections 3306thus can increase the resonant frequency of the scanner 3300.

The use of electronically controlled elements to control resonance in ascanner is not limited to controlling the horizontal scanning frequency.For example, in the embodiment of FIG. 35, a mirror body 3500 hasinterdigitated comb drives 3502 that extend from the body's edges. Combdriven actuators are known structures, being described for example inTang, et al., ELECTROSTATIC-COMB DRIVE OF LATERAL POLYSILICONRESONATORS, Transducers '89, Proceedings of the 5^(th) InternationalConference on Solid State Sensors and Actuators and Eurosensors III,Vol. 2, pp. 328-331, June 1990, which is incorporated herein byreference.

Respective conductors 3504 extend from each of the comb drives 3502 toallow tuning voltages Vtune1, Vtune2 to control the comb drives 3502. Asis known, applied voltages produces lateral forces F1, F2 in the combdrives 3502. Flexible arms 3506 at the distal ends of the comb drives3502 bend in response to the forces F1, F2, thereby shifting the mass ofthe flexible arms 3506 relative to the center of mass 3508 of therespective half of the mirror body. Because the position shift isparallel to the axis of rotation of the mirror body 3500, the horizontalresonant frequency does not shift significantly. However, if thevoltages are set such that the flexible arms experience differentposition shifts, the mirror body 3500 can be made slightly unbalanced.The mirror body 3500 will then begin to approximate the Lissajouspattern of FIG. 20. Adjusting the tuning voltages Vtune1, Vtune2produces a corresponding adjustment in the scan pattern. Where themasses of the flexible portions 3506 and the voltages Vtune1, Vtune2 arechosen appropriately, the resonant frequency of vibrations from theunbalanced mirror body will be an integral multiple of the horizontalscanning frequency and the Lissajous pattern will be stable. Bymonitoring the scan pattern and adjusting the tuning voltages Vtune1,Vtune2 accordingly, the Lissajous pattern can be kept stable. Thus, theelectronically controlled structures can assist in pinch correction.

FIG. 36 shows an alternative approach to controlling the resonantfrequency of a scanner 3600. In this embodiment, the scanner 3600 ishoused on a platform 3602 in a sealed package 3604 having a transparentlid 3606. The package 3604 also contains a gas, such as a helium orargon mix, at a low pressure. The resonant frequency of the scanner 3600will depend, in part, upon the pressure of within the package 3602 andthe properties of the gas, as is described in Baltes et. al., THEELECTRONIC NOSE IN LILLIPUT, IEEE Spectrum, September 1998, pp. 35-39,which is incorporated here by reference. Unlike conventional sealedpackages, the package 3602 includes a pair of outgassing nodules 3610concealed beneath the platform 3602.

The nodules 3610 are formed from an outgassing material, such asisopropanol in a polymer, atop a resistive heater 3611. Electricalcurrent causes resistive heating of the heater 3611, which, in turncauses the nodule 3610 to outgas. An electronic frequency controller3614, controls the amount of outgassing by applying a controlled currentthrough pairs of electrodes 3612 positioned on opposite sides of each ofthe nodules 3610. The increased gas concentration reduces the resonantfrequency of the scanner 3600. For greater frequency variation,absorptive polymer segments 3618 coat the scanners torsion arms 3620 to“amplify” the absorptive effect on resonant frequency.

Typically, the above-described variable or “active” tuning approachesare most desirable for producing small frequency variations. Forexample, such small frequency adjustments can compensate for resonantfrequency drift due to environmental effects, aging, or internal heatbuildup. To reduce the difficulty of active tuning approaches or toeliminate active tuning entirely, it is desirable in many applicationsto “tune” the resonant frequency of a scanner to minimize the differencebetween the scanner's uncompensated resonant frequency and the desiredscan frequency. Such frequency differences may be caused by processingvariations, material property variations, or several other effects.

FIG. 37 shows one approach to tuning the scanner's uncompensatedresonant frequency, in which a scanner 3700 is fabricated with integraltuning tabs 3702A-B, 3704A-B, 3706A-B, 3708A-B, 3710, and 3712.Initially, the scanner's mirror body 3714 and torsional arms 3716 aredimensioned to produce a resonant frequency (with all of the tuning tabs3702A-B, 3704A-B, 3706A-B, 3708A-B, 3710, and 3712 attached) that isslightly below the desired resonant frequency. Once the scanner 3700 isassembled, the resonant frequency can be measured in a variety offashions. For example, the scanner 3700 can be driven in one of thetechniques described previously and the mirror response can be monitoredoptically. Alternatively, impedance versus frequency measurements mayalso provide the resonant frequency relatively quickly.

The determined resonant frequency is then compared to the desiredresonant frequency to identify a desired frequency compensation. Basedupon the identified frequency compensation some of the tuning tabs3702A-B, 3704A-B, 3706A-B, 3708A-B, 3710, and 3712 can be removed, forexample by laser trimming or mechanical force to reduce the mass of themirror body 3714. As is known, lowering the mass of the mirror body 3714(in the absence of other variations) will increase the resonantfrequency. The number and position of the tabs to be removed for theidentified frequency compensation can be determined through modeling orempirical data. Preferably, the removed tuning tabs are positionedsymmetrically relative to the center of mass of the respective half ofthe mirror body and with respect to the axis of rotation of the mirrorbody 3714. To make this symmetricity easier, the tuning tabs 3702A-B,3704A-B, 3706A-B, 3708A-B, 3710, and 3712 are positioned in thesymmetric locations about the mirror body 3714. For example, tuning tabs3702A-B and 3704A-B form a quartet of tabs that would typically beremoved as a group. Similarly, tuning tabs 3710 and 3712 form a pair oftabs that would typically be removed as a pair.

While the tuning tabs 3702A-B, 3704A-B, 3706A-B, 3708A-B, 3710, and 3712in FIG. 37 are shown as equally sized for ease of presentation, it isnot always necessary or even desirable to make them the same size. Insome applications, such tabs may be variably sized to allow greaterflexibility in tuning. In such an embodiment, the sizing of the tabswill be selected to correspond roughly to desired tuning increments.

As is known for MEMs devices, several MEMs scanners 3700 are typicallyformed on a single substrate. It is therefore desirable and somecircumstances to tune the resonant frequency before the substrate isdiced. Advantageously, the MEMs scanners 3700 may be tuned while stillpart of the substrate. In this application, a probe station monitors andoutputs from the piezoresistive position sensors to determine theresonant frequency of the scanner 3700. A microprocessor basedcontroller is coupled to the probe station and determines the differencebetween the measured resonant frequency and the desired resonantfrequency. A software program in the controller then identifies thecorresponding set of tuning tabs 3702A-B, 3704A-B, 3706A-B, 3708A-B,3710, and 3712 and the identified tuning tabs are then removed, forexample, through laser ablation. Because each stage of the tuning can beautomated, the scanners 3700 can be automatically tuned prior to dicingof the substrate.

As shown in FIG. 38A, another approach to tuning the resonant frequencyof the scanners can be performed before the scanners are separated. Inthis approach, several scanners 3850 are located on a wafer 3852.Although only a 18 scanners 3850 are shown on a portion of the wafer3852, one skilled in the art will recognize that the entire wafer 3852may hold many more scanners 3850. Each of the upper six scanners 3850includes a set of distributed masses 3854A-F on an upper surface of itsmirror 3856. (The remaining scanners 3850 include tuning tabs, asdescribed above with reference to FIG. 37.) As described above, theresonant frequency of each of the respective scanners 3850 will dependupon several factors, including the amount of mass that is offset fromthe scanner's pivot axis. The masses 3854A-F will therefore tend toreduce the resonant frequency of the scanners 3850 relative to scannerswithout such masses.

Once the wafer 3852 is processed to the stage shown in FIG. 38A, anelectrical probe station monitors the resonant frequency of each scanner3850. The measured resonant frequency is then compared (either by anautomated test program or by a user) to the desired resonant frequency.The user or a test program on a computer then determines the approximateamount of mass to be removed to shift the resonant frequency to thedesired resonant frequency.

In one embodiment where the masses 3854A-F are formed from thin or thickfilm metals, the determined portions of the masses 3854A-F are removedby laser ablation. Preferably, the removed portions of the masses3854A-F are symmetrically distributed over the face of the mirror 3856to limit off-axis vibration.

In another embodiment, the masses 3854A-F are formed from a materialthat outgases in response to an applied heat, such as certain types ofcommercially available organic polymers. In this embodiment, a heatingcoil is formed on the mirror surface to heat the material or an inputoptical beam can be applied to cause heating. Alternatively, where themirror carries a conductive coil for actuation such as that described inU.S. Pat. No. 5,606,447 to Asada et al. entitled Planar Type MirrorGalvanometer and Method of Manufacture, passing a current through thecoil may provide sufficient heat to cause the material to outgas. Oneskilled in the art will recognize other approaches to removing or addingmaterial to the mirror 3856, such as selectively depositing a materialon the mirror's upper surface. Also, although the process is describedas being incremental, i.e., the removed portion of mass is calculatedthen removed, the process can be continuous. In such an approach, thescanner 3850 is activated and the resonant frequency is monitored whilematerial is removed or added. The material continues to be removed oradded until the measured resonant frequency matches the desired resonantfrequency.

In another approach to frequency tuning, a block 3870 of a migratingmaterial such as a copper or chromium is formed on each torsion arm 3872of scanner 3874. The block can be formed according to conventionalintegrated circuit processing techniques. As is known, such materialstend to migrate into silicon when heated and change the mechanicalproperties of the silicon. Thus, the mechanical properties of thetorsion arms 3872 will change as the material and silicon are heated,for example through optical energy input by heating the scanner 3874.Typically, heating the entire scanner 3874 would be performed after thesubstrate was diced so that the migration could be controlledindependently for individual devices. As the mechanical properties ofthe torsion arms 3872 change, the resonant frequency of oscillation of amirror 3876 changes. As with the general procedure described above withrespect to FIG. 38A, a probe station and electronic or user controlledfabrication station can thus shift the resonant frequency from aninitial value toward a desired value. In both of the approachesdescribed with reference to FIGS. 38A-B, an automatic probe station andmicroprocessor controlled mass removal or heating can automate theprocess of tuning the resonant frequency of the devices on the wafers,as may be desirable in some applications such as very high volumeproduction.

As described above with respect to FIG. 12, tiling in two dimensions canallow a large, high resolution display with less demand upon a scanner.FIG. 40 shows one difficulty that may arise when four separate sources3800, 3802, 3804, 3806 feed a common scanner 3808. As can be seen fromthe ray tracing for the lower left scanner 3800, the upper right source3804 is positioned within an expected scanning field 3810 of the lowerleft source 3800. With no further adjustment, the upper right source3804 would be expected to occlude a portion of the image from the lowerleft source 3800, producing an unilluminated region in the correspondingtile.

FIG. 41 shows one approach in which the effects of overlapping ofsources and beams can be reduced. In this embodiment, light arrivesthrough separate fibers 3900, 3902, 3904, 3906 and is gathered andfocused by respective GRIN lenses 3908, 3910, 3912, 3914 onto respectiveturning mirrors 3916, 3918, 3920, 3922. As is visible for two of themirrors 3916, 3922 in FIG. 41, the turning mirrors 3916, 3922 are verysmall mirrors that redirect light from their respective GRIN lenses3908, 3914 toward a curved, partially reflective mirror 3924. The mirror3924 returns the incident light toward a centrally positioned scanner3926 that scans periodically, as described previously. The scanned lightpasses through the partially transmissive mirror 3924 toward an imagefield 3928 where an image can be viewed.

As can be seen in FIG. 41, the GRIN lenses 3908, 3914 gather diverginglight from the respective fibers 3900, 3906 and reduce the beam width tosubstantially its minimum diameter at the respective turning mirror3916, 3922. The beam 3930 then expands as it travels to the curvedmirror 3924. The curved mirror 3924 converts the expanding beam 3930into a substantially collimated or slightly converging beam 3932 havinga diameter slightly smaller than the mirror width W of the scanner 3926.

It can be seen in FIG. 41 that the turning mirrors 3916, 3918, 3920,3922 will block light from other turning mirrors during a portion oftheir scans. However, because the turning mirrors block only smallsection of the beams and because the beams converge at the image field3924, the effect will be a slight dimming of the corresponding pixel.Uncompensated, this might produce a slight variation from the desiredpixel intensity. However, the programmable gate array 2506 describedabove with respect to FIG. 29 can pre-weight the intensity to offset thedimming effects of the turning mirrors 3916, 3918, 3920, 3922.

To further improve efficiency the display of FIGS. 39 and 41 can alsotake advantage of properties of polarized light. In some applications,the fibers 3900, 3902, 3904, 3906 (or other light sources such as laserdiodes) emit polarized light. A polarization dependent reflector 3934,such as 3M's Dual Brightness Enhancement Film coats the inner surface ofthe mirror and reflects the polarized incident beam 3930. As thereflected beam 3932 travels to the scanner 3926, the beam 3932 passesthrough a quarter wave plate that rotates the polarization by 45degrees. The beam 3932 is then reflected by the scanner 3926 and passesthrough the quarter wave plate once again, so that the polarizationrotates by a total of 90 degrees and is orthogonal to the original beam3930. The orthogonally polarized beam passes efficiently through thepolarization dependent reflector 3934 and travels to the image field3928.

FIG. 42 shows how the use of a tiling approach can reduce raster pinchwithout a correction scanner. In this embodiment, modulated light froman input fiber 4102 enters one or the other of a pair of transmissionfibers 4104, 4106 as dictated by an optical switch 4108. Light exits thetransmission fibers 4104, 4106 and strikes a common scanner 4110 thatscans light from the first fiber 4104 onto a first region 4112 of animage field 4114 and scans light from the second fiber 4106 onto asecond region 4116 of the image field 4114. The fibers 4104, 4106 areoriented so that the first and second regions 4112, 4116 overlap veryslightly in an overlap area 4118.

During forward sweeps of the scanner 4110, an electronic controller 4120activates the switch 4108 so that light passes through the second fiber4106. The scanner 4110 thus redirects the light along a first scan line4122 in the second region 4116. At the end of the forward sweep, thecontroller 4120 activates the switch 4108 so that light now passesthrough the first fiber 4104 and is scanned along a first scan line 4124in the first region 4112. For each subsequent sweep of the scanner 4110,the controller 4120 activates the switch to produce sets of lines ineach of the regions 4112, 4116. Because the vertical scan continuesduring the forward sweeps, the lines may be slightly tilted, as shown inFIG. 42. While such tilt is typically not observable by a viewer, ifdesired, custom optics can produce a “counter”-tilt that offsets thescanning tilt. Alternatively, the image data may be predistorted by theprogrammable gate array 2506 described above with respect to FIG. 29 tocompensate.

This structure is not limited to two horizontal tiles or to a singlelight emitter. For example, as shown in FIG. 43, light from two fiberscan be switched into four fibers to produce a 2-by-2 tiled image.

In this approach, an input fiber 4200 is coupled to four fibers 4202,4204, 4206, 4208 by a set of optical switches 4210, 4212, 4214, whereeach fiber feeds a scanning assembly 4216 from a respective angle. Aswitch controller 4220 activates the switches 4210, 4212, 4214 accordingto the direction of the sweep and according to the tracked location ofthe user's gaze, as provided by a gaze tracker (not shown). The gazetracker may be any known apparatus for determining gaze direction.

For example, when the user looks at the top half of the image, a firstfiber 4206, aligned to produce an image in the upper left tile 4222feeds the scanning assembly 4216 during the forward sweeps. A secondfiber 4208, aligned to produce an upper right tile 4224 feeds thescanning assembly 4216 during reverse sweeps. When the user looks at thelower half of the image, a third fiber 4204, aligned to produce thelower left tile 4226, feeds scanning assembly 4216 during forwardsweeps. A fourth fiber 4202, aligned to produce the lower right tile4228, feeds the scanning assembly 4216 during reverse sweeps. While eachof the fibers 4200, 4206, 4208, 4204 is represented as a single fiber,in some applications each fiber 4200, 4206, 4208, 4204 may actuallyinclude a plurality of fibers 4200, 4206, 4208, 4204. In suchapplications each fiber 4200, 4206, 4208, 4204 is fed by a plurality ofinput fibers 4200 and a corresponding plurality of switch sets. Such anembodiment advantageously allows a plurality of lines to be writtensimultaneously. Writing a plurality of lines simultaneously reduces thefrequency of the horizontal scanner relative to the single line writingapproaches described above, thereby reducing the difficulty of scanning.Also, providing light simultaneously from a plurality of light emittersreduces the amount of light energy required from each source for a givendisplay brightness and reduces the modulation frequency of the beam.This reduces the performance requirements of the light sources, therebydecreasing the cost and complexity of the overall display.

While the embodiments of FIGS. 42 and 43 have been described hereinusing fibers and optical switches, in some applications, discrete lightsources, such as laser diodes, LEDs, microlasers, or gas lasers mayreplace each fiber. In such applications, electrical switches (e.g.,transistors) selectively control drive currents to the respectivesources or control external modulators aligned with the respectivesources to control feeding of light during forward an reverse sweeps ofthe mirror.

Although the invention has been described herein by way of exemplaryembodiments, variations in the structures and methods described hereinmay be made without departing from the spirit and scope of theinvention. For example, the varying resonant frequency approachesdescribed herein, both active and passive, may be applied to a varietyof MEMs devices other than scanners. Such tuning may be useful ingyroscopes, matched filters, opticals switches, or any of a number ofother MEMs applications. Additionally, the approaches to varying themechanical properties of the MEMs material during processing or ofshifting the position or amount of mass relative to a fixed point can beapplied to variety of devices including non-resonant devices.

Similarly, the positioning of the various components may also be variedin the various imaging systems described herein. In one example ofrepositioning, the correction scanners can be positioned in the opticalpath either before or after the other scanners. Also, an exit pupilexpander may be added or omitted in many applications. In suchembodiments, conventional eye tracking may be added to ease coupling ofthe scanned beam to the eye. Moreover, the scanning system can be usedfor projection displays, optical storage and a variety of other scannedlight beam applications, in addition to scanned retinal displays.Further, a variety of other timing control mechanisms, such asprogrammable delays, may be used to compensate for the variable speed ofthe scanner in place of the approaches described with reference to FIGS.24-31. Additionally, in some applications it may be desirable for easeof positioning or for other reasons to use a plurality of scanners, eachof which may be fed by one or more beams. In such a structure, eachscanner and its corresponding light sources produce respective sets oftiles. The overall image is than formed by combining the sets of tilesfrom each of the scanners, either by adjacent positioning or byoverlapping. Although overlapping is generally preferred only where eachscanner is used for a respective wavelength, in some applicationsoverlapping may be used for interlacing or other approaches to imagecombination.

In another alternative approach to timing and distortion correction, thememory map may be undistorted and addressed at a constant rate. Tocompensate for nonlinearity of the scanner, the data for each locationis derived from the retrieved image data and output at fixed increments.Referring to FIG. 27, for example, data would be output at a time 1500,even though this time did not correspond directly to a pixel time. Tocompensate, the buffer 2508 is addressed at the 10^(th) and 11^(th)locations for this line. Then, the output data is a weighted average ofthe data from the 10^(th) and 11^(th) locations. Thus, the buffer 2508is clocked at a constant rate and pixels are output at a constant rate.Yet, by controlling the addressing circuitry carefully and performing aweighted averaging, the output data is sinusoidally corrected. Also,although the light emitters and light sources described herein utilizelaser diodes or LEDs, with or without fibers, a variety of other lightemitters such as microlasers, gas lasers, or other light emittingdevices may desirable in some applications. Moreover, although theexemplary scanning assemblies described herein utilize torsionallymounted mirrors, other scanning assembly structures, such as spinningpolygons, comb drive mirrors, acousto-optic scanners, and other scanningstructures may be within the scope of the invention. Also, while thebeams are shown as converging upon a single scanner, in someapplications it may be desirable to use separate scanners for each beamof light or to use a plurality of scanners that each reflect a pluralityof beams. Accordingly, the invention is not limited except as by theappended claims.

1. A scanning module, comprising: a MEMS scanner that moves through apredetermined scan path at a selected scan rate having a scanningperiod; and a MEMS membrane mounted to the MEMS scanner, the membranebeing deformable through a desired deformation range and having aresponse time sufficiently fast to deform through the desireddeformation range within the scanning period, wherein the scanningmodule has an expected imaging distance from an image plane and whereina plurality of locations on the predetermined scan path each have arespective expected optical path length to the image plane, and whereinthe deformation range of the MEMS membrane is selected to correct fordifferences in the respective expected optical path lengths.
 2. A methodfor correcting wave front aberrations comprising: emitting a beam from alight source along a beam path; reflecting the beam from a deformablemirror; actuating a scanner to which the deformable mirror is mountedalong a predetermined scan path at a selected scan rate having ascanning period, the scanner causing an aberration in the beam, theaberration varying with movement of the scanner along the predeterminedscan path; and actuating the deformable mirror mounted to the scanner ata frequency and amplitude effective to reduce the aberration withmovement of the scanner along the predetermined scan path.
 3. The methodof claim 2, wherein actuating the deformable mirror comprises applying avoltage to an electrode positioned beneath the deformable mirror andsubstantially coextensive with a movable portion of the deformablemirror.
 4. The method of claim 2, wherein actuating the deformablemirror comprises parabolically deflecting the deformable mirror.
 5. Themethod of claim 2, wherein actuating the deformable mirror comprisesdeflecting the deformable mirror according to a fourth order polynomial.